- Add --session-dir CLI flag to specify custom session directory
- SessionManager API: second param of create(), continueRecent(), list(), open()
changed from agentDir to sessionDir (direct directory, no cwd encoding)
- When omitted, uses default (~/.pi/agent/sessions/<encoded-cwd>/)
- --session now derives sessionDir from file's parent if --session-dir not provided
- list() validates session header before processing files
- Closes#313
Co-authored-by: scutifer <scutifer@users.noreply.github.com>