feat: native mentions in Chat, Tasks and Knowledge

[Maneek21]

Adds one @ picker for people, agents, tasks and wikis across Chat, Tasks and Knowledge: messages/replies, task descriptions/comments and wiki bodies. Tasks and wikis open as links and show authorized backlinks. Human Send/Post or explicit Notify mentions publishes eligible notifications; document agent mentions create passive attention. Existing explicit chat invocation and shared conversation storage retain their owners.

Core owns identity, authorization, current reference state, saved-hash publication and durable delivery. Existing manual/module relationships are preserved. Defty and bound employee MCP tools discover exact tokens, read authorized source context and consume their own attention. Agent writes reject unavailable targets before proposal/approval creation and again at execution. Resource scopes are opt-in and do not broaden existing credentials. Bounded parsing excludes literal/quoted content.

Validation and evidence

• Fresh PostgreSQL 16 + browser: 21/21 native/agent/catalog cases passed, including concurrent delivery. Original evidence: 16 screenshots and 3 session recordings on a0cd499d, desktop/recipient/mobile, live socket, save/navigation/Notify flows and model-prefix rendering.
• Full CI: Passed; 1,474 API tests passed, 8 skipped, zero failed. Type checks, web lint/contracts, production build, supported upgrade and production-image browser smoke are included. Hermes release certification is skipped by repository release-scope rules.
• Security and CodeQL alert gate passed with no new alerts.
• Live OpenAI acceptance on 638de7e: gpt-5.4-mini, 8/8 passed, 24 actual requests. Real Defty runner and real authenticated MCP handlers cover grounded references/facts, governed writes, passive attention, private/tenant boundaries, changed permissions, own acknowledgement, approval queues and renamed records. Synthetic fixtures cleaned; key never persisted. Sanitized traces are in the local review report outside Git. Earlier failing runs are retained; this does not certify every model or a deployed third-party runtime. Final a0cd499 only narrows wrapping for unrelated MCP tools; native handlers and guidance are unchanged. The opt-in harness is apps/api/test/live-native-mentions.ts.
• Local focused compatibility: 40 passed, one PGlite concurrency skip; shared codec suite 80 passed.

Excluded: Notes, Calendar, Canvas, Email and custom App Kit records. Feature defaults off; DEFT_NATIVE_MENTIONS_ENABLED=true enables the pilot. Additive preview.61 migration avoids the separate Platform/Email candidate's reserved .31–.60; combined manifests/checksums and joint release checks remain an integration gate. UI recordings use offline test identities; live provider behavior has separate transcript evidence.

Closes #351.