docs: add comprehensive keyring integration documentation (#314)

Add docs/features/keyring-integration.md covering:
- Secret reference syntax (${keyring:name}, ${env:NAME})
- CLI commands: secrets set/get/del/list/migrate with all flags
- Web UI: secrets management page
- REST API: 5 endpoints under /api/v1/secrets/
- Platform support (macOS Keychain, Linux, Windows)
- Auto-restart, log sanitization, troubleshooting
- Security considerations

Co-authored-by: Claude Code <noreply@anthropic.com>

Author: Dumbris