feat(chat): backport fragment feedback and selection actions to main

[Yuqing (mydmdm)]

Summary

User-authorized Main backport of the Alpha-validated #294 and #295 for #293. The branch starts at main commit ef481056f090f9065c2456ad13a461204a956b51 and contains exactly two cherry-pick -x commits:

Original Alpha commit	Main backport commit	Change
ac0517bb78abaaf1241ee387a7e4ca2d485f7524	ce6f92fa168274e896fe6c14b00123f69c195096	Fragment comments rendered into the owning thread input
24eaf878d40af401ed3d2887cd1d8182c478b753	f69bb4a024bc5e250b6bd9b73c0cc5b25cd4b5f7	Selected-text Copy / Add as note / Comment icon actions

No unrelated Alpha commits are included. The original Alpha branches/history are not rewritten or changed. The only cherry-pick conflict was adjacent paragraphs in docs/architecture/preview-workspace.md; resolved by retaining Main's existing empty-chat disclaimer documentation alongside the feature documentation. Main's existing disclaimer implementation and tests remain intact.

Alpha-to-Main promotion compatibility

The user requires this backport not to make subsequent Alpha promotion harder. Compared Git's merge result before and after the backport against the same exact refs:

• Main baseline: ef481056f090f9065c2456ad13a461204a956b51.
• Alpha baseline: 10d48793c925f795ef3ed67a3e249ae5c3b81d76.
• Before: git merge-tree --write-tree --name-only origin/main origin/alpha.
• After: git merge-tree --write-tree --name-only fix/issue-293-main origin/alpha.
• Both simulations report exactly the same sole conflicting file, docs/architecture/preview-workspace.md; no new conflict is introduced. Existing promotion Promote Alpha to Main #283 is already conflicting before this backport.
• The two simulated result trees differ only in that document's conflict representation. Retaining both existing paragraphs and normalizing blank-line separation produces identical document content; all other merged files are byte-identical. The feature paragraphs merge automatically after the backport, leaving only the pre-existing Main disclaimer paragraph decision.

Thus, for these verified branch heads, this backport introduces no additional promotion conflicts or different final content. This is not a guarantee against future independent changes to either branch. Do not use an ours merge, drop Alpha commits, reset branches, or rewrite promotion history to mark the feature as delivered. A normal merge commit preserves the recorded backport history and source attribution. This PR does not merge or modify #283.

Behavior and scope

Select text within one completed assistant Markdown surface to Copy selected plain text, save selected Markdown as a Note, or Comment and explicitly Add to input. Multiple comments and free-form input combine in the thread draft before ordinary Send. Localized template labels, tooltips, clipboard retry, snapshot semantics and source provenance remain as validated on Alpha.

Whole-response bottom actions remain unchanged. No streaming comments, default presets, permanent annotations, new APIs/dependencies/storage, unrelated UI changes or release. No live-provider validation or manual cross-browser verification is claimed.

Validation on Main

• Repository pnpm typecheck, pnpm format, pnpm lint:fix, pnpm check:i18n and git diff --check passed; formatting/linting left the backport worktree clean. Lint reports 0 errors and 350 existing warnings.
• Targeted Vitest: 12 files / 116 tests passed, including Main's existing message-list behavior plus selection/Markdown, clipboard/retry, note payload/provenance, draft isolation, comment flow, Milkdown accessibility/chrome/drag/roundtrip, composer, history and submission-controller regressions.
• Production fixture build passed.
• Production Playwright configuration: all 9 tests passed, covering all selection-action/comment tests and the existing activation/windowing/completion/eviction/performance regressions.
• Frozen dependencies restored only in the new worktree after the validation command reported missing Vitest; no manifest, lockfile or subtree source changes.