fix: skip per-command exit logs after a shutdown signal

[pintu544]

Fixes #255

Problem

After pressing Ctrl-C, every command logs exited with code SIGINT. When the shell reclaims the terminal before teardown finishes (e.g. the npm/yarn wrapper exits first), those late lines land on the fresh prompt and visually corrupt it — the prompt only reappears after another Ctrl-C or any command that writes to stdout. All reporters describe the same recovery pattern, which points at late output clobbering the prompt rather than a dead terminal.

Fix

Once KillOnSignal catches SIGINT/SIGTERM/SIGHUP, the per-command exit log lines are noise — the user requested the shutdown, so the exits are expected (the codebase already remaps post-SIGINT exits to code 0 for the same reason). This change skips just those log lines:

• KillOnSignal accepts an optional onSignal callback, invoked synchronously when a signal is caught, before commands are killed.
• LogExit accepts an optional isShuttingDown predicate; close events arriving while it returns true are not logged.
• lib/index.ts wires the two together with a shared flag. Both new options are optional, so the public API stays backwards compatible.

Exit codes, kill behavior, and shutdown ordering are unchanged — closes that happen before the signal are still logged, and normal (non-signal) exits log exactly as before.

Verification

• New unit tests: LogExit skips logging while shutting down / logs when not; KillOnSignal invokes onSignal.
• Updated bin/index.spec.ts: the SIGINT test now asserts exit code 0 and the absence of the exit log line.
• Full suite: 647/647 tests pass; eslint and tsc --noEmit clean.
• Manual end-to-end: group SIGINT on two signal-trapping children → exit code 0 with no late output (previously printed both exited with code SIGINT lines); normal echo/exit 3 run still logs exits and returns code 1.

[coveralls]

coverage: 98.836% (+0.006%) from 98.83% — pintu544:fix/skip-exit-logs-after-signal into open-cli-tools:main