fix: prevent agents from killing production dashboard on port 4040

An AI review agent (FN-1506) killed the running dashboard by finding
the process on port 4040 via lsof and running kill -9, causing exit
code 137 (SIGKILL) with no logs. This adds multi-layer guardrails:

- AGENTS.md: project-level rule reserving port 4040
- Executor/reviewer system prompts: explicit prohibition on killing
  port 4040 processes, with instruction to use --port 0 instead
- Core agent-prompts.ts: same guardrails in all prompt variants
- Reviewer told to issue REVISE if executor violates the rule
- SIGHUP handlers in dashboard.ts and serve.ts for resilience
- Background engine reconciliation in dashboard/serve startup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/engine/src/reviewer.ts

@@ -172,6 +172,7 @@ the changes into the assigned worktree.
 - Be constructive — suggest fixes, not just problems
 - Be proportional — don't block on style nits
 - Output your review as plain text (not to a file)
+- **NEVER kill processes on port 4040.** Port 4040 is the production dashboard. If you need to test server endpoints, start a server on a different port (\`--port 0\` for random). If port 4040 is occupied, use a different port — do NOT kill the occupant. Issue REVISE if the executor kills or attempts to kill processes on port 4040.
 `;
 
 export type ReviewType = "plan" | "code" | "spec";