fix: graceful SIGINT/SIGTERM signal handling + 22 tests

[diberry]

fix: graceful SIGINT/SIGTERM signal handling

Adds signal handlers to prevent orphaned processes when users press Ctrl+C.

Problem

The CLI had no signal handlers. Pressing Ctrl+C during long operations (shell sessions, watch mode, remote bridge) could leave child processes running indefinitely.

Changes

3 files, 379 lines added:

1. packages/squad-cli/src/cli-entry.ts — Top-level safety net handler

   • Registers SIGINT/SIGTERM handlers early in bootstrap
   • 3s cleanup timeout (unref'd so it doesn't block exit)
   • Double-signal force-exits immediately
   • Exit codes: 130 (SIGINT), 143 (SIGTERM)

2. packages/squad-cli/src/cli/shell/index.ts — Shell-specific handler

   • Calls Ink's unmount() to flow through existing cleanup (session close, client disconnect, telemetry shutdown)
   • Ensures readline interface is properly closed

3. test/cli/signal-handling.test.ts — 22 tests

   • Exit code verification (130/143)
   • Double-signal force-exit behavior
   • 3s cleanup timeout with unref()
   • Handler registration validation
   • Shell-specific unmount flow

Risk

Low. Only adds behavior on signal — doesn't change any existing code paths. Follows existing patterns in the repo. Build passes clean.

[diberry]

@bradygaster — Critical fix: CLI had no signal handlers — Ctrl+C during shell/watch/serve could orphan child processes. 44 lines of code + 22 tests. Low risk, high impact.

[bradygaster]

EECOM review: Signal handling fix looks solid. Top-level safety net + shell-specific unmount path are both well-structured. 22 new tests cover exit codes, double-signal force-exit, cleanup timeout, and handler registration. Rebased on dev (includes #483 timeout fix + #480 race fix). CI failure was pre-existing platform-adapter timeouts — now resolved by the rebase.