docs(adr): Context Architecture v2 — lifecycle, projection, typed memory (#3192)

[mabry1985]

Summary

Governing ADR (0107) for the Context Architecture v2 initiative, the gate for all implementation work under #3184.

• D1: Separate durable state from model-visible context. The projection pipeline composes at request time without checkpointing derived frames, eliminating stale-snapshot accumulation in conversation history.
• D2: Typed memory axes on one physical backend. Four new columns (memory_kind, subject, review_state, expires_at) provide orthogonal classification while domain stays as a topic bucket.
• D3: Four context layers with stable ordering. Stable prefix, session-frozen snapshot, current-turn delta, on-demand retrieval — ordered for maximum prefix-cache hit rate.
• D4: One semantic runtime context contract. Named delivery adapters per surface (lead, subagent, ACP, goal, scheduler, plugin, incognito) with documented injection/persistence/trust policies.

Also includes:

• Phase 0 characterization fixtures (tests/test_context_characterization.py, 11 tests) documenting today's Chunk fields, compose_context return contract, and context_frame_message shape
• Changelog fragment

Resolves #3192 · Tracker: #3184

Test plan

• ruff check . passes
• python -m pytest tests/test_context_characterization.py -q — 11 passed
• Human review of ADR content and invariant definitions

🤖 Generated with Claude Code

https://claude.ai/code/session_01WEMxBi71vjtmmmziFCMcby

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[mabry1985]

Duplicate — work consolidated on #3204.