feat(email-writing): build authorized thread context

[seonghobae]

Stack dependency

This Draft PR is stacked on #1328 (feat/llm-email-writing-review-evidence-task3) and implements Task 4 only from the committed LLM email-writing implementation plan.

Customer next action

This PR is context-construction only. Customers should continue to write and send email with the current editor and send path. No writing-guidance feature, language profile, or model is available in product. Writing and sending stay on the current path.

Included

• server-authoritative selected-email lookup through Email.owner_filters(auth_context.user_id, auth_context.organization_id);
• tenant-indistinguishable unavailable-email outcomes for missing, deleted, cross-user, and cross-organization references;
• canonical persisted thread-key lookup with the same owner scope and defensive row revalidation;
• immutable EmailWritingContextBundle, chronological message records, and participant-role records;
• complete selected source, quoted material, signatures, sender/reply-to/recipient metadata, source timestamps, reply objective, current draft, and declared language tag;
• explicit prompt trust labels separating untrusted email content from untrusted authored content;
• deterministic complete-message selection by thread membership, chronology, review mode, recency, and size budget;
• no string, UTF-8, grapheme, selector, or JSON truncation: oversized selected content fails closed and older related messages are omitted only as complete units with stable limitation codes;
• canonical message de-duplication, owner/thread filtering, malformed-ID rejection, and bounded candidate/message counts;
• recipient and reply-target roles derived solely from persisted server headers;
• no browser-supplied thread text, participants, recipient roles, endpoint, credential, or semantic selector.

Semantic-authority boundary

The service performs authorization, thread membership, chronology, bounded context construction, recipient-role derivation, and trust labeling only. It does not decide which message is important by keywords, infer tone, quality, or actionability, use sender domains or recipient counts as semantic features, or apply nearest-text or positional fallback.

Exact-head verification

Previous head (exact current PR head at launch):

0376d0ac896f6ea86183234495184694b27d7896

GitHub last showed base feat/llm-email-writing-review-evidence-task3 at stale:

6ab000efd8ad262e3f51462ae5cfddd57d849c53

Current head (normal merge commit, not squash, not rebase, not force-push):

4570747ccebd57ccaab30ffc68239f0c9d2f1ca0

Live parent #1328 (feat/llm-email-writing-review-evidence-task3) merged as the second parent:

51fb5e8543247b1e5c790f3fdf98424c8fbed669

Normal merge commit parents:

• first parent: 0376d0ac896f6ea86183234495184694b27d7896
• second parent: 51fb5e8543247b1e5c790f3fdf98424c8fbed669

Live parent #1328 already contains #1327 fb7c406ee1328a6ac42dbaf54bb6852c199d8b0a / #1322 bfc2df112136bb9fe358778d701e78bf9e78b685 / develop@dd8d15191338b841f9e6f3a06507c6a5643b95d0.

ADR numbering follows the live parent:

• calendar ADR-0004 stays unique (0004-status-weighted-calendar-conflicts.md);
• email-writing remains ADR-0005;
• Task 4 context files were unchanged by the merge (no product conflict, no context-semantics edit).

Local validation counts on 4570747ccebd57ccaab30ffc68239f0c9d2f1ca0:

• authorized email/thread context tests: 18 passed (tests/test_email_writing_context_service.py)
• context terminal-boundary / coverage tests: 14 passed (tests/test_email_writing_context_service_coverage.py)
• combined focused context + terminal-boundary tests: 32 passed
• parent contract sanity tests: 8 passed (tests/test_email_writing_contracts.py + tests/test_email_writing_contracts_terminal_coverage.py)
• combined focused + parent contract tests: 40 passed
• context coverage: 100% (254 statements, 72 branches, fail-under=100)
• ruff: All checks passed
• production-module compileall for the context service: ok
• git diff --check: ok
• conflict markers / product context-semantics edits: 0
• stability scan: no Timeout / Fatal / Warn / Denied

Predecessor evidence does not transfer. Checks, reviews, and security evidence recorded for 0376d0ac, 6ab000ef, or any earlier head are non-passing for this head.

Merge boundary

Keep this PR Draft while #1328 and its parent stack remain unmerged. This slice does not add contextual-orchestrator transport, tenant inference credentials, model or Judge calls, prompt templates, API routes, editor UI integration, sending, publishing, or release changes. Independent review, zero actionable threads, branch protection, and exact-current-head required checks remain mandatory before merge. This update does not approve, merge, squash, empty-requeue, force-cancel, or mark Ready.

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📥 Commits

Reviewing files that changed from the base of the PR and between 51fb5e8 and 4570747.

📒 Files selected for processing (4)

• .github/workflows/email-writing-context-tdd.yml
• backend/services/email_writing_context_service.py
• backend/tests/test_email_writing_context_service.py
• backend/tests/test_email_writing_context_service_coverage.py

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[seonghobae]

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[github-actions]

PR governance metadata gate is not ready for 4570747ccebd57ccaab30ffc68239f0c9d2f1ca0:

• Draft PR: merge automation is paused.

[seonghobae]

@coderabbitai review

Please review the unchanged exact current head 4570747ccebd57ccaab30ffc68239f0c9d2f1ca0 (normal merge of live parent #1328 51fb5e85). Predecessor 0376d0ac evidence does not transfer. Local Task 4: 32 focused context tests passed, 100% coverage. Keep Draft. This comment is not an approval.

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@seonghobae I will review the exact current head 4570747ccebd57ccaab30ffc68239f0c9d2f1ca0. The PR remains Draft. This action does not approve or merge the PR.

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