[Bugfix #389] Fix: Kill shellper processes during af cleanup

[waleedkadous]

Summary

Fixes #389

Root Cause

af cleanup relied exclusively on the Tower REST API (TowerClient.killTerminal()) to kill shellper processes. This silently fails when:

• Tower is not running (exception caught and swallowed)
• The terminal was already removed from Tower's registry (e.g., process exited naturally)
• Tower was restarted and lost the shellper session mapping

In all these cases, the shellper process and its PTY children (Claude, bash) are left running as orphans.

Fix

Added killShellperProcesses(worktreePath) in cleanup.ts that:

1. Searches ps -ww -eo pid,args output for shellper-main.js processes
2. Matches by exact worktree path in the JSON config cwd field (prevents false positives from partial path prefixes)
3. Kills via process group signal (SIGTERM to -pid) to also terminate PTY children
4. Falls back to individual PID kill if process group kill fails
5. Runs after the Tower API kill attempt as a reliable fallback for all non-shell builders
6. Non-fatal: silently handles cases where ps isn't available or no processes are found

Test Plan

• Added regression test (cleanup-shellper-kill.test.ts, 9 tests)
• Tests cover: exact cwd matching, no false positives on different/partial paths, multiple kills, process group fallback, ps failure handling, self-process exclusion
• Existing cleanup tests pass (cleanup-task-builders.test.ts, 13 tests)
• TypeScript type-check passes
• Build passes