Settle pending turns when providers exit before turn start

[DevVig]

Problem

When a user submits a turn, the server persists client/turn/requested and
marks the thread active before the provider emits turn/started.

A provider can acknowledge the turn/start RPC and then exit before emitting
that first lifecycle event. At that point the command has succeeded, so there
is no command failure to reconcile, but activeTurnId is still null. Existing
provider-exit handling treated the thread like an idle resident session and
emitted nothing, leaving it visibly Working with no provider process alive.

Lifecycle contract

The runtime now includes its existing pendingTurnStart state in the final
per-thread snapshot captured before process-exit cleanup. On an unexpected
provider exit, the host daemon applies these rules:

1. An active turn follows the existing turn-scoped completion and error path.
2. A pending turn start with no active turn emits the existing thread-scoped
   provider_process_exited error.
3. An idle session emits nothing.
4. An expected process exit emits nothing.

The server already treats provider_process_exited as run.failed, so the
thread moves from active to error, pending interactions are interrupted,
and parent-thread notification behavior remains consistent with other failed
turns.

Relationship to #1432

#1432 settles prompts when a provider returns a terminal signal without first
starting a turn. This PR covers the complementary case where the provider
process disappears and no terminal signal can arrive.

Safety and non-goals

• The behavior is provider-independent, although the reported failure was
  observed with Claude Code.
• It relies on an actual unexpected process exit; it does not add an inactivity
  timeout, watchdog, or general stuck-turn reconciler.
• pendingTurnStart is set before dispatch and cleared by turn/started,
  turn/completed, a terminal provider error, command failure cleanup, or
  thread cleanup. This distinguishes the vulnerable window from an idle
  resident session.
• No persisted event schema or database model changes are required.

Wire compatibility

HOST_DAEMON_PROTOCOL_VERSION is bumped to 116 because an updated daemon
can now emit a failure event for a pre-turn/started process exit.

Verification

• Real child-process regression: turn/start succeeds, the provider exits
  before turn/started, and the exit snapshot retains
  pendingTurnStart: true.
• Host-daemon event construction and event transport are covered.
• Server ingestion confirms provider_process_exited transitions an active
  thread to error and preserves parent-notification behavior.
• @bb/agent-runtime: 941 tests passed across 46 files.
• @bb/host-daemon: 548 tests passed across 46 files.
• @bb/host-daemon-contract: 49 tests passed across 3 files.
• @bb/server: 1,471 tests passed across 162 files.
• Turbo typechecks passed for all four packages above.
• Prettier and git diff --check passed.

Jonathan Borgwing authored the original diagnosis and implementation. The
rebased branch preserves both original commits and credits Jonathan on the
post-ack regression hardening commit.

[Copilot]

Pull request overview

Fixes a reconciliation gap where a provider process can exit after a client turn request activates a server thread but before the provider emits turn/started, leaving the live command pending until timeout. The change plumbs the runtime’s pending-turn-start state into the provider-exit snapshot and synthesizes a thread-scoped terminal error on unexpected provider exits during that window.

Changes:

• Extend the runtime provider-exit per-thread snapshot with pendingTurnStart and plumb it through captureThreadExitState.
• In host-daemon reconciliation for unexpected provider exits, emit a thread-scoped system/error (provider_process_exited) when pendingTurnStart is true and there is no active turn.
• Add runtime and host-daemon regression tests covering the pre-turn/started provider-exit window and update existing fixtures to include the new snapshot field.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/agent-runtime/src/types.ts	Adds pendingTurnStart to the per-thread provider-exit snapshot type.
packages/agent-runtime/src/runtime.ts	Captures pendingTurnStart from the runtime’s pending-turn-start tracking set in exit snapshots.
packages/agent-runtime/src/runtime.process-lifecycle.test.ts	Adds coverage asserting exit snapshots include pendingTurnStart: true when the provider dies mid turn-start.
apps/host-daemon/src/runtime-manager.ts	Synthesizes a thread-scoped terminal error when the provider exits unexpectedly before turn/started.
apps/host-daemon/src/runtime-manager.test.ts	Adds regression test for the pre-start crash window and updates existing exit fixtures.
apps/host-daemon/test/command/thread-dispatch.test.ts	Updates process-exit fixture to include pendingTurnStart.
apps/host-daemon/src/app.test.ts	Updates onProcessExit test fixtures to include pendingTurnStart.

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