feat: add parallel runtime controls and recovery

[dickymoore]

This is the second PR in the parallel execution stack. It is opened as a draft to avoid accidental merge before the MVP branch lands.

Please review/merge order:

1. fix: codex loop robustness #2: Codex bug fixes.
2. feat: add parallel story execution MVP #4: parallel execution MVP.
3. This PR: runtime controls and recovery.
4. Safety/modularization follow-up.

Scope:

• Adds controller PID/control-file reporting.
• Adds runtime control commands: pause, resume, drain, and stop.
• Adds graceful signal handling for long-running controller sessions.
• Adds worker/provider idle timeout handling so stalled work does not block the full run forever.
• Adds retry/salvage context from previous kept worker attempts.
• Preserves sprint tracker formatting during status updates.
• Adds script snapshotting so edits to the repo copy do not affect a live controller/worker run.

Tested:

• bash -n on core, wrappers, and installer.
• Wrapper --help smoke test.
• Fake Codex parallel smoke test with RALPH_CONCURRENCY=2 in a temporary git project: two ready-for-dev stories ran through worker worktrees, fake dev/code-review workflows completed, controller integrated both worker outputs, and both stories ended as done in sprint status.

No live Claude/Codex agent workflow was invoked; the provider was faked to avoid spending real model calls during branch validation.