feat(http): X-Session-Id fallback + drop workspace suffix

[manojp99]

Summary

Changed

• X-Session-Id header is now recognized as a fallback for the existing X-Client-Session-Id correlation mechanism (PR feat(http): session resume via X-Client-Session-Id; client-authoritative reconciliation #71). opencode and other Vercel AI SDK-based clients send X-Session-Id by default; amplifier-agent now picks it up automatically, so session-resume + client-authoritative reconciliation works for opencode with zero config. X-Client-Session-Id remains authoritative when both headers are present.

• Workspace name is no longer suffixed with the client session id. Previously, X-Client-Session-Id: abc would route requests into workspaces/<base>-abc/. Now the workspace stays at <base> and per-client distinction is purely at the session_id level (workspaces/<base>/sessions/http-abc/). This keeps workspace-level hook state (context-intelligence, etc.) shared across all sessions of the same server process, where it belongs.

How we discovered this

Live traffic inspection of opencode 1.17.9 → amplifier-agent confirmed the following incoming headers:

x-session-affinity: ses_10dba803effekTqUujLAFyL9me   ← opencode's session id
x-session-id:       ses_10dba803effekTqUujLAFyL9me   ← opencode's session id (Vercel AI SDK)

opencode sends X-Session-Id (and X-Session-Affinity with the same value) automatically via the Vercel AI SDK's provider-utils. It does not send X-Client-Session-Id (our convention from PR #71), does not populate the OpenAI user field, and there's no plugin hook in opencode to mutate request headers.

A three-line server-side change reading X-Session-Id as a fallback unlocks the entire session-resume + client-reconciliation chain from PR #71 for every opencode user with zero config.

The workspace-suffix change is a follow-on cleanup: each opencode session creates its own sessions/http-<sid>/ directory, which is sufficient per-session distinction. Putting the sid in the workspace name was fragmenting workspace-level hook state (context-intelligence, hooks config) per opencode session — not what we want.

Changes

• Reverted temporary DEBUG INCOMING instrumentation block from chat_completions.py
• chat_completions.py: fallback header read + simplified workspace logic (no suffix)
• test_reconciler.py: updated workspace slug assertion from test-ws-{sid} → test-ws
• test_session_id_fallback.py (new): 4 tests covering X-Session-Id-only, precedence, no-header fallback, and workspace-not-suffixed scenarios
• CHANGELOG.md: documented both changes under [Unreleased] ### Changed

Test evidence

tests/http/test_reconciler.py::* — 8/8 passed
tests/http/test_session_id_fallback.py::* — 4/4 passed
tests/config/ + tests/http/ + tests/cli/ — 312 passed (1 pre-existing failure in test_provider_sources unrelated to this PR)

Hard constraints honored

• X-Client-Session-Id clients continue to work identically — header takes precedence when both are set
• Wire protocol unchanged — request, response, headers, body are all unmodified
• CLI face untouched (single_turn.py, _runtime.py)