Fix custom agent path handoff

[jamilahmadzai]

Summary

• stop the companion recorded in launcher state when MONK_AGENT_PATH changes
• preserve the existing bounded stop guarantee before launching the replacement
• add deterministic POSIX and Windows regressions to the install workflow

Root cause

The launcher compared a saved PID against the newly selected executable. During an A-to-B custom path switch, that ownership check rejected the still-running A process, so B could exit on the occupied endpoint while the launcher reported A as ready.

The launcher now reads the prior executable from its saved state and validates the saved PID against that path before stopping it.

Validation

• all shell regression tests pass on macOS
• shell syntax checks pass
• all three POSIX launcher copies are byte-identical
• all three PowerShell launcher copies are byte-identical
• workflow YAML parses successfully
• Install E2E passes on Ubuntu, macOS, and Windows for the published commit

The upstream run is awaiting the repository approval required for first-time contributors.

Fixes #270