Fix owned process handle audit regressions

[brickfrog]

What this PR does:

• Adds pgid_is_alive(pgid) in src/sys using kill(-pgid, 0), treating EPERM as alive and pgid <= 1 as false.
• Switches recovery liveness to probe the recorded process group rather than the group leader pid.
• Keeps interpret_kill_agent defaults pure/inert and has dispatch inject real pgroup kill/pidfile deletion.
• Extends hermetic /tmp native pgroup coverage for leader-exited/descendant-alive.

Audit citations:

• Closes audit finding 1 from PR Replace /proc process-table scanning with owned process-group handles in kill_agent #573 task contract: remove_pid_file default called @sys.delete_file_sync directly from src/tools/shutdown.mbt instead of using dispatch-injected deletion.
• Closes audit finding 2 from PR Replace /proc process-table scanning with owned process-group handles in kill_agent #573 task contract: exec recovery liveness probed a recorded process-group id with pid_is_alive / kill(pgid, 0), missing live descendants after the leader exits.
• Closes audit finding 3 from PR Replace /proc process-table scanning with owned process-group handles in kill_agent #573 task contract: pgroup tests only covered the leader-alive happy path.

Verification:

• moon test --target native
• moon build src/bin/choir_lint --target native && _build/native/debug/build/src/bin/choir_lint/choir_lint.exe
• ! grep -rn "list_pids_with_cwd_prefix|mcp_stdio_pids|read_proc_process_table_entries|process_table_ppid_from_proc_stat|ProcessTableEntry" src/
• ! grep -rn "choir_list_pids_with_cwd_prefix" src/sys/stub.c

Note: no .choir/audit-receipts directory was present in this leaf worktree, so the audit references cite the PR #573 task contract text.

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon test --target native
  • exit: 0
  • head: fe1f87f
  • output tail:

Total tests: 2008, passed: 2008, failed: 0.

• moon build src/bin/choir_lint --target native && _build/native/debug/build/src/bin/choir_lint/choir_lint.exe
  • exit: 0
  • head: fe1f87f
  • output tail:

Finished. moon: no work to do