Orchestration V2: open hardening work #15013

These items come from apps/server/src/orchestration-v2/TODO.md and the TODO(interrupt-hardening) comment in Orchestrator.ts. The wave 1 CI and docs PR removes both, so this issue now tracks them. Each item was checked against main at 4f7760e. Items that are done are not listed.

Interrupts

• Define and test what happens when a provider accepts an interrupt but then completes normally. The run keeps a run_interrupt_request with no result.
• Make an explicit policy, with a test, for starting a new root turn while an interrupted provider turn is still active.
• Add projection tests where the provider delays or ignores an interrupt.

Projections

• Add a projection test where promoting a queued message to a steer succeeds. Today only the rejection is tested.

Context transfer

• Give unresolved context transfers typed failure reasons: missing source point, unsupported capability, context too large, source not stable, adapter resolution failure. Today there is one failed status with free-text error.

Capabilities

• Call optional adapter methods (injectHistory, compactThread) through a capability check, not ! assertions (ProviderTurnStartService.ts).

Rollback

• Decide what rollback does when the thread has no active provider thread, and add a test.

Subagents

• Add a test for forking from a subagent thread.

Dev tooling

• scripts/mobile-showcase-environment.ts still seeds V1 tables. V2 clients do not read them after startup, so the App Store showcase threads probably do not show (not checked on a device). Seed the orchestration_v2_projection_* tables and check on an iOS simulator.

Filed by Claude Code (Opus 5.5).