feat(opencode): OpenCode ephemeral sessions - #43656

AI code review — automated review for reference; please use your judgment.

• packages/core/src/session/projector.ts:102-110 — the liveness heartbeat only fires in applyUsage, so an ephemeral session whose run sits idle-but-alive for a long stretch (slow tool, pending permission ask, human-in-the-loop pause) never refreshes time_updated and can be reaped by the startup sweep while its owning process is still running. Either emit heartbeats from non-usage activity too, or make sure the documented staleness window comfortably exceeds the maximum legitimate idle time.
• packages/opencode/src/cli/cmd/run.ts:465-482 — removeEphemeral() has no timeout; a hung session.delete HTTP call will block every completion path (return done()) and the SIGINT/SIGTERM handlers, leaving the CLI wedged at exit exactly when it is trying to clean up. Wrap the fan-out in a short timeout (e.g. Promise.race with 5-10s) — the server-side sweep already covers stragglers.
• packages/opencode/src/share/session.ts:41-43 — create() skips auto-share for ephemeral sessions but says nothing about child sessions: creating a sub-session under an ephemeral parent still persists the child, which then outlives the deleted parent with a dangling parentID. Either inherit ephemeral to children or refuse child creation under an ephemeral parent.
• packages/core/src/session/projector.ts:108 — nit: embedding Date.now() directly in the SQL CASE bypasses the app's date abstraction used everywhere else (DateTime.makeUnsafe(...) on read); routing it through the shared helper keeps clock handling uniform and makes time-based tests easier to fake.
• packages/opencode/test/session/session.test.ts / core/test/session-projector.test.ts — good coverage of flag plumbing and decoding; the two riskiest behaviors (startup sweep reaping abandoned sessions, heartbeat keeping live ones) appear untested — a pair of focused tests there would lock in the semantics this design leans on.

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