feat(http): session resume via X-Client-Session-Id; client-authoritative reconciliation

[manojp99]

What this PR does

When the chat-completions client sends X-Client-Session-Id, amplifier-agent now uses a deterministic http-<client_sid> as the amplifier session_id, auto-detects whether this is the first turn or a continuation by checking if the session state dir exists on disk, and passes is_resumed to the existing kernel resume mechanism (same primitive the CLI face's --resume flag uses). One opencode conversation = one amplifier session — unified audit trail, persistent hook state across turns, append-mode events.jsonl.

Client-authoritative transcript reconciliation in src/amplifier_agent_http/_reconciler.py. Since the chat-completions wire is stateless and the client sends full history every turn, on divergence between stored and incoming the client wins by fiat — we persist the client's view over our stored copy without any rewind ceremony. Sufficient for opencode and any well-behaved OpenAI-compatible client. No new event types introduced.

Changes

File	What changed
src/amplifier_agent_http/_reconciler.py	New — HTTP-face-private reconcile_client_history(). Persists client messages to SessionStore, returns messages unchanged.
src/amplifier_agent_http/routes/chat_completions.py	Initializes client_session_id_clean before the workspace block; adds sid + is_resumed determination; calls reconciler; passes session_id + is_resumed to run_chat_turn.
src/amplifier_agent_http/_session_runner.py	Adds is_resumed: bool = False to run_chat_turn signature; threads it through to prepared.create_session().
tests/http/test_reconciler.py	New — 8 tests covering reconciler unit behavior and route integration.
CHANGELOG.md	Added two entries under [Unreleased] > Added.

Backward compatibility

Clients that do not send X-Client-Session-Id get a random sid per turn (via run_chat_turn) and is_resumed=False — identical to current behavior.

How to smoke test

1. Start the server: amplifier-agent serve chat-completions --api-key test
2. Send turn 1 with the header:

   curl -s -X POST http://localhost:8000/v1/chat/completions \
     -H 'Authorization: Bearer test' \
     -H 'Content-Type: application/json' \
     -H 'X-Client-Session-Id: my-smoke-test' \
     -d '{"model": "<your-model>", "messages": [{"role": "user", "content": "Say hi"}]}' | head -5

3. Confirm a session dir was created:

   ls ~/.amplifier-agent/state/workspaces/*-my-smoke-test/sessions/http-my-smoke-test/
   # Should show: transcript.jsonl  metadata.json

4. Send turn 2 with the same header and different content. Check that the stored transcript was updated (client wins).
5. Without the header, confirm the session dir is not created (legacy path unchanged).

Validation

• uv run pytest tests/http/ — 24 passed (16 pre-existing + 8 new)
• uv run pytest tests/config/ — 47 passed
• uv run pytest tests/cli/ — 237 passed, 1 pre-existing env-var failure on main (unchanged)
• uv run ruff check src/ tests/ — clean
• uv run pyright src/ — 1 pre-existing error in single_turn.py (not in this PR's files)