fix(antigravity): kill stale monk-agent PID in ensure hooks (#394)

[ZacLou]

Closes #394

The PreInvocation ensure hooks (.antigravity-plugin/hooks/ensure-monk-agent.ps1 and .sh) now kill a stale monk-agent process recorded in their PID file before spawning a replacement, matching the cleanup already performed by scripts/start-monk-agent.{ps1,sh}.

• PowerShell: adds Stop-StaleAgent and calls it after the PID file path is resolved.
• POSIX shell: adds stop_stale_agent and calls it before starting the binary.
• Adds regression tests for both paths.

[NyxSpecter4]

Reporter of #394 here — verified: the ensure hooks now mirror Stop-ManagedAgent (table on #394). The PID-reuse guard (/proc/<pid>/exe on Linux, ps -o comm= elsewhere, $OldProcess.Path on Windows) is the right shape; thanks for not killing a bare PID.

Two notes:

1. tests/ensure-monk-agent-kill-stale-pid.sh on Linux (Ubuntu, sh = dash): the stale-kill assertion holds, but the final check fails with replacement monk-agent process (<pid>) is not running. Cause is in the harness, not the fix: the fake monk-agent is a copy of sh, so monk-agent serve --host … tries to open a file named serve in the hook's cwd, not the serve stub on PATH. A one-line wrapper script that exec sleeps regardless of args fixes the test.
2. Stop-StaleAgent elseif (Test-Path $ExpectedPath) { $KillIt = $true } — when the OS gives no Path (access denied / 32-vs-64-bit), this kills the PID as long as the expected binary exists on disk, which is almost always. Consider $OldProcess.ProcessName -like 'monk-agent*' as the fallback instead, matching the .sh side's comm= check.

Branch is based on v0.1.59; main is v0.1.60 — rebase to keep the diff to 4 files.

[ZacLou]

@NyxSpecter4 thanks for the review. Pushed both fixes:\n\n1. The shell test now runs the hook from $fake_bin so the fake binary (copy of sh) finds the serve stub in its cwd and the replacement process stays alive.\n2. Stop-StaleAgent now falls back to $OldProcess.ProcessName -like 'monk-agent*' instead of Test-Path $ExpectedPath, matching the POSIX side's comm= check and avoiding killing unrelated reused PIDs.\n\nPlease take another look.

[NyxSpecter4]

As the reporter of #394, I took another look at the updated head (7cd3668):

1. Shell regression test — ran tests/ensure-monk-agent-kill-stale-pid.sh on Ubuntu (sh = dash): passed. Stale process killed, replacement recorded in the PID file and running. The fake_bin cwd fix resolves the harness issue I flagged.
2. PowerShell — code review only, since I have no Windows runner here: Stop-StaleAgent now falls back to $OldProcess.ProcessName -like 'monk-agent*' when the OS provides no process path, matching the POSIX side's comm= guard. A bare PID is never killed. I could not execute the .ps1 test in this environment, so that one remains unverified by me.
3. Rebase — branch is based on v0.1.60 (8d8efce); the diff is exactly the four hook/test files (356 insertions, no unrelated changes).

Both notes from my earlier review are addressed, and the PID-reuse guard is preserved on both platforms. Approving from the reporter's side — merging is of course up to the maintainers. Thanks for the quick turnaround on both points.

[NyxSpecter4]

Both fixes verified: Stop-StaleAgent now falls back to ProcessName -like 'monk-agent*' only when no path is available (a reused PID is never killed unless the name matches — consistent with the POSIX comm= guard), and the $fake_bin cwd fix resolves the harness failure.

Ran tests/ensure-monk-agent-kill-stale-pid.sh on Linux/dash: exit 0, "stale PID kill test passed". LGTM — good to merge.

[NyxSpecter4]

Approving: both requested fixes verified in code; tests/ensure-monk-agent-kill-stale-pid.sh run on Linux/dash -> exit 0, "stale PID kill test passed". Note: .ps1 regression statically reviewed only (no pwsh available here); Stop-StaleAgent path-match + name-fallback logic is sound.