refactor(context): remove prepare lifecycle

[HugoSkylight]

Summary

Removes the legacy context prepare lifecycle and leaves recall on the probe/plan/execute search path.

Changes included:

• /api/v1/context now supports commit/end only and rejects phase=prepare.
• Removed ContextManager prepare orchestration, prepare cache, prepare response DTOs, client helper, selected-skill bookkeeping, and prepare-side owner/autophagy bookkeeping.
• Kept commit/end lifecycle behavior intact.
• Added session_id to the typed /api/v1/memory/search result item so the new search contract preserves conversation traceability.
• Updated tests to assert /memory/search as the recall contract instead of context prepare.

Validation

• uv run --group dev ruff format --check .
• uv run --group dev ruff check .
• uv run --group dev pytest tests/test_context_manager.py tests/test_http_server.py tests/test_http_client.py tests/test_noise_reduction.py tests/test_phase2_shrinkage.py -q
• uv run --group dev pytest tests/test_eval_contract.py -q
• uv run --group dev pytest -q (1268 passed, 28 skipped, 1 warning)