feat(core): resume suspended sessions after service restart

[kitlangton]

Addresses #35646

Summary

• add a private session.time_suspended nullable timestamp with a partial index, following the table's existing time_compacting / time_archived idiom; it records suspension, not Session status, and stays out of public Session.Info
• add SessionRestart, an inert core service with two actions: suspendActiveSessions and resumeSuspendedSessions
• only the managed server (serve --service) invokes the actions: resume is forked on startup and suspend runs as a teardown finalizer, after connections close and before execution teardown interrupts drains
• clear stale suspension through EventV2 live commit hooks on execution started/succeeded/failed; interruption preserves it so the suspend-then-interrupt teardown ordering holds; replay never recreates or destroys suspension
• consume each suspension atomically (compare-and-clear returning the row) so a Session resumes at most once; resumed drains overlap with bounded concurrency
• migration adds the nullable column with no historical backfill; old shutdown events never arm new work
• fold SessionExecutionLocal back into SessionExecution as the main layer

Design doc: specs/v2/session-restart-continuation.md. At-most-once by choice: losing one automatic continuation after a crash mid-handoff is safer than retry loops over ambiguous provider/tool work. Hard-crash recovery remains out of scope (#35642).

Verification

• bun run test -- test/session-execution.test.ts test/session-projector.test.ts test/database-migration.test.ts (packages/core, 40 tests)
• bun run test -- test/service.test.ts (packages/cli: election + suspension consumed + one session.execution.started)
• bun run migration --check (packages/core)
• bun typecheck in core, server, cli, and opencode
• file-scoped Prettier and oxlint
• pre-push full repository typecheck: 31 packages passed