Show agents with hooks installed in `partio status`

[jcleira]

partio status currently shows Agent: claude-code from the config file, but this reflects configuration intent rather than the actual installed state. Users who edit configs or run partio enable multiple times can end up with a mismatch between config and reality.

Add an "Agents:" line to the status output showing which agents actually have hooks installed on disk, by reading existing hook files and checking for the partio marker (isPartioHook() in internal/git/hooks/hooks.go).

Desired output

Status:     enabled
Strategy:   manual-commit
Agent:      claude-code
Hooks:      installed
Agents:     claude-code
Checkpoints: branch exists

Line is omitted when partio is disabled or no partio-owned hooks are found.

Program

See: .minions/programs/show-installed-agents-in-status.md

Inspired by: entireio/cli#847