Add session digest command and optional exit hook wrapper

[dmoliveira]

What changed

• Added scripts/session_digest.py with:
  • run mode to capture a session digest (timestamp, reason, cwd, git branch/status snapshot)
  • show mode to print the latest digest
  • optional post-digest hook command via --hook
• Added scripts/opencode_session.sh wrapper:
  • launches opencode
  • traps process exit (including Ctrl+C) and writes digest automatically
  • optionally runs hook command after writing digest
• Added OpenCode commands in opencode.json:
  • /digest
  • /digest-run
  • /digest-show
• Updated installer to chmod and surface digest commands
• Updated README with usage and exit-wrapper examples
• Extended self-tests and install smoke tests to cover digest flows

Why

You asked for both manual and automatic session-end behavior. This provides:

• manual on-demand command (/digest run)
• automatic on process exit (opencode_session.sh) with optional hook command

Validation

• make validate
• make selftest
• make install-test