Document musing-ai-service in AGENTS.md and add ai-service.mdc rule

[danibsheehan]

Summary

`AGENTS.md` and `musing-project.mdc` had zero mention of `service/` despite it being a whole second application in this repo by now (8 merged PRs across the AI second-brain build) — any agent working here had no idea it existed, that it's a separate Node/TS backend with its own toolchain, or its conventions. This is the last item from the original architecture plan (`docs/ai-service-integration`).

• New `.cursor/rules/ai-service.mdc` (scoped to `service/**`): the fail-fast required-env pattern, the two separately-billed/separately-budgeted providers, `requireAuth` as the real access boundary (not GCP IAM), `requireBudget` needing to gate `embed-page` too (correcting the original design doc, which only called out LLM routes), the rate limiter, the `ALLOWED_ORIGINS` comma-escaping gotcha in `deploy-service.yml`, the two locked-down pgvector RPC functions, local dev, and that `service/` needs its own lint/test/build.
• `AGENTS.md`: intro paragraph, a Conventions table row, two new Constraints (don't assume the AI service is configured; don't assume root checks cover `service/`), a Configure pointer to `VITE_AI_SERVICE_URL`.
• `musing-project.mdc`: thin pointer section, matching its own "don't duplicate long content" style.
• README: `VITE_AI_SERVICE_URL` was missing from the Configuration table; `readme.mdc`'s own Configuration bullet was stale too — both fixed.

Note on `/code-review`: it surfaced two real findings, but on `SidebarSearch.tsx` — code from the already-merged #94, not part of this diff (the review scanned the whole repo since this branch has no code diff). Handling those separately, as their own fix, rather than bundling an unrelated bug fix into a docs PR.

Test plan

• `check_stack_docs.py`, `npm run format:check` — pass
• Docs-only change, no code paths affected

🤖 Generated with Claude Code

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PR guide

Auto-generated from changed paths. Fill in Summary and How to verify in the PR description above.

Touches: docs / agent guidance, other

Diff: 5 files changed, 93 insertions(+), 4 deletions(-)

Suggested verify

• N/A - docs/tooling only; confirm locally if anything user-facing changed

Checklist (applies to this PR)

• No unintended secrets or local-only config committed

Reviewer focus

• Scope looks docs- or tooling-only; confirm there is no hidden runtime impact

Commits

• 4f6b011 Document musing-ai-service in AGENTS.md and add ai-service.mdc rule

Files by area

Area	Files
docs / agent guidance	4
other	1

CI

Primary check: CI runs lint, Prettier format check, Vitest coverage, and build for every PR. Coverage tables are posted separately.

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Unit tests (Vitest)

File	Coverage	Lines	Branches
All files	55%	62%	48%	✅
src/components/DatabaseEmbedNodeView.tsx	0%	0%	0%	✅
src/components/DatabaseTableView.tsx	0%	0%	0%	✅
src/components/SlashMenu.tsx	72%	85%	60%	✅
src/context/WorkspaceContext.tsx	25%	35%	14%	✅
src/extensions/blockIdOnBlocks.ts	75%	100%	50%	✅
src/extensions/musingDatabaseEmbed.ts	70%	90%	50%	✅
src/extensions/singleTopLevelBlock.ts	80%	100%	60%	✅
src/hooks/usePageIndexing.ts	95%	90%	100%	✅
src/lib/aiClient.ts	81%	86%	76%	✅
src/lib/blockEditorCommands.ts	42%	50%	35%	✅
src/lib/blockPlainText.ts	55%	60%	50%	✅
src/lib/editorBlockText.ts	13%	13%	14%	✅
src/lib/emojiSuggestionItems.ts	93%	100%	87%	✅
src/lib/htmlToDocx.ts	24%	31%	18%	✅
src/lib/pageToExportHtml.ts	97%	100%	94%	✅
src/lib/resolveWikiPage.ts	91%	100%	83%	✅
src/lib/slashMenuOptions.ts	57%	57%	0%	✅
src/lib/themePreference.ts	32%	25%	40%	✅
src/lib/workspaceStorage.ts	80%	81%	80%	✅
src/lib/pageDocument/blockGutterOps.ts	92%	100%	84%	✅
src/lib/pageDocument/blockIdAtSelection.ts	83%	100%	66%	✅
src/lib/pageDocument/lastBlockNeedsBelowHit.ts	91%	100%	83%	✅
src/lib/pageDocument/serializeDocToBlocks.ts	58%	69%	47%	✅
src/lib/pageDocument/slashMenuDeleteRange.ts	82%	95%	70%	✅
src/lib/pageDocument/tryDeleteEmptyTopLevelBlock.ts	87%	94%	81%	✅

Minimum allowed coverage is 0%

Generated by 🐒 cobertura-action against 4f6b011