Add generic agent support with customizable command directories (#1639)

- Add --ai generic option for unsupported AI agents (bring your own agent)
- Require --ai-commands-dir to specify where agent reads commands from
- Generate Markdown commands with $ARGUMENTS format (compatible with most agents)
- Rebuild CHANGELOG from GitHub releases (last 10 releases)
- Align version to 0.1.3

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>