feat: /agents command to list/select agent profiles at runtime

[VforVitorio]

What problem does this solve?

Users cannot inspect or switch agent behaviors quickly during a session. Mode switching (�sk/auto/strict) controls permissions, but not reasoning style or task specialization.

Proposed solution

Add /agents slash command family:

• /agents -> list available profiles (default, planner, implementer, reviewer, docs)
• /agents use -> switch active profile for current session
• /agents show -> show profile capabilities and constraints

Profiles should define:

• System-prompt extension
• Preferred tool set
• Output style constraints

This enables faster adaptation without restarting chat.

Alternatives considered

Expose profile selection only through config file. Rejected because in-session switching is much more practical during iterative development.