Address PR #1327 review: harden selectors, merge changelog, add screenshots

- CorpusChat.tsx: use crypto.randomUUID() for SYNC_CONTENT fallback messageId;
  add aria-label to BackButton so tests can target it stably
- CreateCorpusActionModal.tsx: export DEFAULT_MODERATOR_INSTRUCTIONS so the
  CT test imports it instead of duplicating the string
- CorpusAgentManagement.ct.tsx: replace fragile .last() on Save Changes with
  role-based locator; add docScreenshot
- CorpusChat.ct.tsx: replace XPath preceding::button selector with aria-label
  lookup; clarify that the title-filter debounce test is coverage-only
- CreateCorpusActionModal.ct.tsx: import DEFAULT_MODERATOR_INSTRUCTIONS from
  source; comment the intentional invalid-trigger cast; add docScreenshot
- CorpusDescriptionEditor.ct.tsx: add docScreenshot
- CHANGELOG: merge duplicate ### Fixed sections into one block under ### Added

Author: JSv4

CHANGELOG.md

@@ -7,10 +7,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-### Fixed
-
-- **`CorpusChat` dropped `SYNC_CONTENT` messages from the visible chat** (Issue #1276, `frontend/src/components/corpuses/CorpusChat.tsx:468-495`): The `SYNC_CONTENT` WebSocket frame is a standalone, non-streaming assistant reply used for synchronous server responses. `ChatTray` (document chat) appends these directly to its `chat` state; the corpus-level chat only forwarded the content to `handleCompleteMessage`, which stores sources in `ChatSourceAtom` but never pushes a message to the visible list. As a result, any `SYNC_CONTENT` the backend sent over the corpus socket rendered nothing. Fixed by mirroring the `ChatTray` pattern — push a new complete assistant message into `chat` before persisting sources/timeline. New regression test in `frontend/tests/CorpusChat.ct.tsx` ("SYNC_CONTENT renders a complete message immediately") pins the behavior.
-
 ### Added
 
 - **Coverage: raise Corpus Chat & Agent Management component tests** (Issue #1276): added 36 new Playwright CT tests across the four lowest-ROI corpus components to drive coverage toward the ≥60% target. Breakdown:
@@ -20,6 +16,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - `frontend/tests/CorpusDescriptionEditor.ct.tsx` (+7 tests): save failure (`ok: false`), save network-error path, reapply of snapshot-less version, twice-click collapse, Cancel Version Edit reset, fetch-md URL failure, and version-count pluralization.
 
 ### Fixed
+
+- **`CorpusChat` dropped `SYNC_CONTENT` messages from the visible chat** (Issue #1276, `frontend/src/components/corpuses/CorpusChat.tsx:468-495`): The `SYNC_CONTENT` WebSocket frame is a standalone, non-streaming assistant reply used for synchronous server responses. `ChatTray` (document chat) appends these directly to its `chat` state; the corpus-level chat only forwarded the content to `handleCompleteMessage`, which stores sources in `ChatSourceAtom` but never pushes a message to the visible list. As a result, any `SYNC_CONTENT` the backend sent over the corpus socket rendered nothing. Fixed by mirroring the `ChatTray` pattern — push a new complete assistant message into `chat` before persisting sources/timeline. New regression test in `frontend/tests/CorpusChat.ct.tsx` ("SYNC_CONTENT renders a complete message immediately") pins the behavior.
 - **Backend CI `changes` job failed on every push to main** (`.github/workflows/backend.yml`): `dorny/paths-filter@v3` was declared without a prior `actions/checkout`, on the stated premise that the action "fetches diffs via the GitHub API". That is only true for `pull_request` events; on `push` events the action shells out to `git branch --show-current`, which fails with `fatal: not a git repository` when the workspace is empty. Every push to a protected branch therefore produced a red X on the `changes` job, silently masked by `continue-on-error: true` plus a `|| github.event_name == 'push'` fail-open gate on downstream jobs. Scoped the `changes` job to `pull_request` events (`if: github.event_name == 'pull_request'`) — where paths-filter actually works — and rewrote the downstream gates on `linter` / `pytest` as `if: always() && (github.event_name == 'push' || needs.changes.outputs.backend != 'false')` so the skip cascade from a `needs`-target that is now intentionally skipped on push does not block the real test jobs. Behaviour preserved: push always runs `linter` + `pytest`; PRs with no backend changes still skip both; PRs where the filter errors transiently (outputs.backend == '') still fail open.
 - **Frontend CI tippy-debug step removed** (`.github/workflows/frontend.yml`): Dropped the `Debug - Check for tippy references` step in the `lint` job — a leftover investigation artifact that ran on every PR/push and produced noise with no diagnostic value today. Flagged in Issue #1319.
 - **Codecov-notify `matching[0]` relied on undocumented API ordering** (`.github/workflows/codecov-notify.yml`): The cross-workflow coordinator picked the "most recent" workflow run per expected name by indexing `matching[0]` on the filter result, leaning on the GitHub Actions REST API returning results newest-first. That ordering is not guaranteed. Added an explicit `created_at` descending sort before taking `matching[0]`, so re-run detection is correct regardless of API-side quirks. Flagged in Issue #1319.

docs/assets/images/screenshots/auto/corpus--agent-management--agent-row.png

@@ -466,12 +466,12 @@ export const CorpusChat: React.FC<CorpusChatProps> = ({
             setIsProcessing(false);
             break;
           case "SYNC_CONTENT": {
-            // SYNC_CONTENT is a standalone, non-streaming assistant message.
-            // Append it to chat so it renders, then persist sources/timeline.
+            // SYNC_CONTENT is a standalone (non-streaming) assistant reply — unlike the
+            // ASYNC path, it must be appended to `chat` directly or it will never render.
             setChat((prev) => [
               ...prev,
               {
-                messageId: data?.message_id || `asst_${Date.now()}`,
+                messageId: data?.message_id || crypto.randomUUID(),
                 user: "Assistant",
                 content,
                 timestamp: new Date().toLocaleString(),
@@ -1007,6 +1007,7 @@ export const CorpusChat: React.FC<CorpusChatProps> = ({
         {isConversation && (
           <ChatNavigationHeader>
             <BackButton
+              aria-label="Back to conversation list"
               onClick={(e) => {
                 e.preventDefault();
                 e.stopPropagation();

docs/assets/images/screenshots/auto/corpus--chat--conversation-list.png

@@ -71,7 +71,7 @@ const DEFAULT_MODERATION_TOOLS = [
   { name: "unpin_thread", description: "Unpin a pinned thread" },
 ] as const;
 
-const DEFAULT_MODERATOR_INSTRUCTIONS = `You are a thread moderator for this corpus. Your role is to:
+export const DEFAULT_MODERATOR_INSTRUCTIONS = `You are a thread moderator for this corpus. Your role is to:
 1. Monitor discussion threads and messages for policy compliance
 2. Take appropriate moderation actions when needed
 3. Respond helpfully to user questions when appropriate

docs/assets/images/screenshots/auto/corpus--chat--empty.png

@@ -8,6 +8,7 @@ import {
   DELETE_AGENT_CONFIGURATION,
   UPDATE_AGENT_CONFIGURATION,
 } from "../src/graphql/mutations";
+import { docScreenshot } from "./utils/docScreenshot";
 
 const TEST_CORPUS_ID = "corpus-aam-1";
 
@@ -152,6 +153,8 @@ test.describe("CorpusAgentManagement", () => {
     await expect(page.getByLabel("Edit agent")).toBeVisible();
     await expect(page.getByLabel("Delete agent")).toBeVisible();
 
+    await docScreenshot(page, "corpus--agent-management--agent-row");
+
     await component.unmount();
   });
 
@@ -604,8 +607,7 @@ test.describe("CorpusAgentManagement", () => {
 
     // Click Save Changes (modal footer button)
     await page
-      .locator(".oc-modal button:has-text('Save Changes')")
-      .last()
+      .getByRole("button", { name: "Save Changes", exact: true })
       .click();
 
     await expect(page.getByText("Agent updated successfully")).toBeVisible({