Add configurable post-session hook for tests and lint

[dmoliveira]

What changed

• Added scripts/post_session_command.py to configure post-session behavior:
  • status, help
  • enable, disable
  • set command <shell-command>
  • set timeout <ms>
  • set run-on <exit|manual|idle> (comma-list)
• Extended scripts/session_digest.py:
  • new --run-post option
  • loads config from ~/.config/opencode/opencode-session.json
  • executes configured post-session command with timeout
  • records post-session result inside digest JSON (attempted, exit_code, timed_out)
• Updated scripts/opencode_session.sh to call digest with --run-post on exit/Ctrl+C
• Added OpenCode command bindings:
  • /post-session, /post-session-help, /post-session-enable
  • /digest-run-post
• Updated docs/install/test flows:
  • README usage and config docs
  • installer self-check and command hints
  • selftest coverage for post-session command execution
  • install-test smoke includes post-session status and digest run-post

Why

This provides a native way to run your preferred test/lint command automatically at session end (including Ctrl+C) and optionally on manual digest runs.

Validation

• make validate
• make selftest
• make install-test