Follow-up to the canonical-session fix: with no explicit session,
every request minted a fresh x-opencode-session, and upstream free-tier
quota is accounted per session. That burns through quota and surfaces
as 429 FreeUsageLimitError with growing reset-after delays, while the
real CLI reuses one long-lived session per conversation.
- Stable canonical session per downstream identity (connectionId, else
auth-header hash, else shared default), evicted after
MEMORY_CONFIG.sessionTtlMs like the other session stores.
- Deterministic x-opencode-request per message (stable across retries,
like the CLI user message id); valid downstream ids preserved.
- 6 more unit tests (22 total).
0c6ab4f99b