Phase 10C: Add contextual goals and structured clarification

[paullizer]

Summary

• Add bounded contextual planning over the current user message and at most two preceding active user turns, with opaque planner refs rebound to exact authorized documents.
• Require one explicit durable choice before prior user text can cross an external retrieval boundary; rebuild and minimize queries from exact user turns at decision, resume, and execution.
• Add one durable server-authored clarification checkpoint with allowlisted questions/options, ETag claims, response leases, claim-generation fencing, exact Retry/Edit recovery, and process-loss reconciliation.
• Add accessible prior-goal disclosure and clarification controls, plus privacy projection across history, collaboration, metadata, SSE, and telemetry.

Security

• Assistant, system, tool, evidence, citation, workspace, deleted, masked, generated, inactive, and foreign content cannot become contextual goal input or external query text.
• Source turns are reauthorized from the exact conversation partition during proposal load, decision, resume, worker entry, and immediately before retrieval or execution.
• Contextual declines remove external selected/automatic closure for the turn without changing saved toolbar state.
• Clarification and response ownership is ETag- and claim-generation-fenced across concurrent requests, Retry/Edit, terminal output, cleanup, and process restart.
• Server-only source IDs, hashes, queries, resume state, clarification linkage, and response hashes are projected out of all browser and collaboration surfaces.

Validation

• Focused Phase 10C security/lifecycle suite: 119 passed.
• Cumulative Phase 9/10A/10B/10C gate: 349 passed, 11 expected environment-gated skips.
• Full-file security gate: BAC 17 files, XSS 19 files.
• Final added-line security scans: BAC 23 tracked Python files, XSS 25 tracked supported files.
• Untracked full-file scans before commit: BAC 6 files, XSS 7 files.
• Route policy, JavaScript syntax, application diagnostics, and tracked/untracked whitespace passed.
• Final targeted independent review found no remaining medium-or-higher issue in the reviewed Phase 10C defect areas.

Documentation

• Bumped the application to 0.250.076.
• Updated CHAT_TURN_ORCHESTRATION.md and release notes for v0.250.076.

Refs #1021