Add agents command to list available agent identifiers

[albertodebortoli]

Description

• Adds a new luca agents subcommand that prints a formatted table of all known agent identifiers and their project skill paths.
• This lets users discover valid values for the --agent flag when installing skills.

Type of Change

• Feature

How Has This Been Tested?

• Manually tested locally (luca agents, luca agents --help)
• Tested on macOS (arch: arm64)
• Swift code builds locally (swift build)
• Tests pass locally (swift test) — 343 tests passing

Screenshots / Demo

$ luca agents
Id              Project Skills Path
──────────────  ───────────────────
adal            .adal/skills
amp             .agents/skills
claude-code     .claude/skills
cursor          .agents/skills
...

Checklist

• Swift code builds locally (swift build)
• Tests pass locally (swift test)
• Code style / formatting respected
• PR title follows conventional style

Breaking Changes?

• No

Additional Notes

Registered under the existing "Listing" command group alongside installed.