fix: preserve Stop-hook continuation after Channel B completion

[Trecek]

A Channel B transcript marker can arrive while Claude is running a blocking Stop hook. Managed lifecycle runs could treat that marker as completion authority, SIGTERM the live worker before its continuation, and report the termination as natural_exit.

Remove Channel B candidate state so completion eligibility starts only from native Channel A terminal evidence. Preserve admitted Channel B corroboration after an observed exit, require an observed exit for NO_KILL, and label synchronous timeout kills as infrastructure kills.

Regression coverage invokes the real required-join Stop dispatcher and checks blocked continuation, approved completion, natural exit, timeout/cancellation cleanup, PTY behavior, pending-child deferral, and ScheduleWakeup deferral. AST guards pin completion-candidate ownership to the native terminal watcher. Existing process and session-binding test helpers are shared across the coverage.

Validation:

• pre-commit run --all-files passed.
• Configured task test-local-gate: 47,551 passed, 613 skipped, 29 xfailed.
• Original issue probe: continuation ran, return code 0, Channel A confirmation.
• All three planned mutations were detected by their intended tests and restored.

The local gate excludes tests/execution/test_process_channel_b.py; its existing CI shard remains the verification gate for that file.

Closes #5283.