refactor: enforce one task contract per turn

[2002yy]

Scope

Seal TaskContract as one immutable per-turn decision before G10-C2.4.

• classify a new turn once from persisted learning state and optional explicit override
• reuse the original persisted contract for continuations and retries
• pass the same contract through routing, external-data policy, pedagogy evaluation, pedagogy planning, route snapshots, and pedagogy snapshots
• accept an explicit task_intent override through the chat API
• remove frontend re-interpretation of the persisted contract from learning_state
• retain backward-compatible fallback classification for direct legacy calls outside the production preparation path
• update the unique project status source and move the next code slice to G10-C2.4

Root cause

The production flow previously classified task intent independently in routing, pedagogy evaluation, and pedagogy planning. The frontend then performed a fourth inference from the returned learning state. These classifiers used different active-learning signals, so one turn could have conflicting task semantics.

Invariants

• new turn: explicit override > explicit text markers > active-learning inheritance > safe quick-answer default
• continuation/retry: persisted parent contract wins; request text or a new override cannot mutate the original turn semantics
• downstream layers consume a contract; they do not silently replace it
• the frontend displays the server-owned contract exactly as persisted
• unknown API override values are rejected before preparation begins

Deliberate boundary

This PR does not add a composer task selector, integrate chat with durable ResearchRun, change closure endpoints, or implement G10-C2.4 PR review context. The API capability is complete; the visual task selector remains a documented UI gap.

Validation

GitHub Actions CI #670 on head 84c7c97798786b57cbc570e57daf74816f2af475 passed:

• pytest: 702 passed
• Ruff: passed
• package helper: passed
• detect-secrets: passed
• incremental mypy baseline gate: passed
• frontend Vitest: passed
• TypeScript project build: passed
• Vite production build: passed