Docs: surface the reranker + close v2.7.0 doc gaps (tags, MCP tool list/resources/prompts) + guardrail test #51

Summary

Close the documentation gaps that accumulated across the v2.7.0 features — starting with the reranker (built but never surfaced in the user-facing setup) — and add a test-style guardrail so the MCP doc surface can't silently drift again.

Context

Several features shipped with code + tests + green CI but incomplete docs (see the new docs/LEARNINGS.md entry "Feature PRs ship code + tests but skip the doc surface"). A sweep of README / docs/*.md / book/src/ found:

• Reranker (Improve LLM-as-reranker quality (prompt + score-parsing robustness) #21 / Improve LLM-as-reranker quality (prompt + score-parsing robustness) #36) — not surfaced for users. It's in the reference docs (CONFIGURATION.md, INTEGRATIONS.md, DECISIONS.md) and book/src/configuration.md, but absent from book/src/local-only.md (the "going fully local" chapter) — the natural place to tell a local user the reranker is an optional local quality boost (qwen3:4b) and how to turn it on. README only says "optional LLM reranking" with no setup or model recommendation.
• journal tags (journal tags: list, count, and rename tags for corpus hygiene #33) — undocumented anywhere (README, docs/, book/src/ all zero).
• MCP tool list is stale. The server now exposes 13 tools (search recent decisions threads show capture meetings todos done stats today ask synth), but the README integrations line still says "search/recent/decisions/meetings" and book/src/integrations/claude-desktop.md (and the Claude Code chapter) list only ~6 — missing show, capture, done, stats, today, ask, synth.
• MCP resources (MCP: expose journal content as resources (today, recent, project indexes) #37) — undocumented (journal://today|recent|projects/{slug}/index appears in no user doc).
• MCP prompts (MCP: expose pre-built prompts (weekly reflection, decisions review, project status) #44) — not in the user-facing MCP/integration docs.

(Note: #46 did document the MCP synth tool in docs/SYNTHESIS.md — that one followed through and is the model to copy.)

Acceptance Criteria

• Reranker is surfaced in book/src/local-only.md (optional local quality boost; reranker: qwen3:4b; the extra model/RAM cost) and in the README's local-setup guidance — not just the reference docs.
• journal tags is documented: book/src/ (a tags/hygiene section), docs/USAGE.md, and a README mention.
• MCP tool list refreshed everywhere to the full current set: README integrations line, book/src/integrations/claude-desktop.md + claude-code.md tables, and docs/INTEGRATIONS.md.
• MCP resources (MCP: expose journal content as resources (today, recent, project indexes) #37) and prompts (MCP: expose pre-built prompts (weekly reflection, decisions review, project status) #44) documented in the integration chapters.
• Guardrail test: a unit test asserts every tool/resource/prompt registered in cmd/mcp.go appears in the README tool list and the integration chapter — so this drift fails CI next time.
• mdbook build clean (no broken links); existing tests pass; lint passes; review approved.

Scope

In scope: README.md, docs/USAGE.md, docs/INTEGRATIONS.md, book/src/local-only.md, book/src/integrations/*.md, a new tags doc section, and the guardrail test (cmd/mcp_test.go or a small docs-coverage test).
Out of scope: new features or behavior; restructuring the book; the reference docs/*.md that are already accurate.

Technical Notes

Key files:

- book/src/local-only.md                 (add the reranker)
- book/src/integrations/claude-desktop.md, claude-code.md   (refresh tool tables; add resources + prompts)
- README.md                              ("exposes search/recent/decisions/meetings" → full list; reranker setup)
- docs/USAGE.md, docs/INTEGRATIONS.md    (journal tags; MCP tool enumeration)
- cmd/mcp.go                             (source of truth for tool/resource/prompt names)
- cmd/mcp_test.go                        (add the coverage assertion)

Patterns to follow:

• Copy the approach #46 used for the synth tool in docs/SYNTHESIS.md (it documented the new MCP surface properly).
• The guardrail test reads the registered names from cmd/mcp.go (or a shared list) and greps the doc files — keep it pure-Go, no network, CI-safe under -p 1.

Complexity

• complexity:medium — several files + one new test; no architectural decisions.

Agent Readiness

• bounded · [x] measurable (each surface named) · [x] context (the audit) · [x] patterns linked (#46 as the model) · [x] no external blockers