feat: support team member management and cleanup team references on agent removal

[mokizzz]

Summary

This PR adds team member management commands and fixes stale team references when removing an agent.

It introduces team add-agent and team remove-agent, and updates agent remove to detect team memberships and clean them up safely (including leader reassignment or empty-team deletion when needed).

This PR was completed with assistance from gpt-5.3-codex.

Changes

• Added new team CLI commands:
• tinyclaw team add-agent <team_id> <agent_id>
• tinyclaw team remove-agent <team_id> <agent_id>
• Added leader-safe removal flow for team remove-agent:
• prompts for a new leader if the removed member is current leader
• prevents removing the last team member via remove-agent (requires team remove for full deletion)
• Enhanced agent remove <agent_id>:
• checks whether the agent belongs to any teams
• prompts user that team membership will be removed together
• removes the agent from all related teams after confirmation
• auto-reassigns team leader when needed
• removes team if it becomes empty
• Updated help/usage text in CLI for new team commands
• Updated documentation:
• README.md
• docs/TEAMS.md
• docs/AGENTS.md

Type of Change

• Bug fix
• New feature
• Enhancement to existing feature
• Breaking change
• Documentation update
• Refactor / code cleanup

Testing

• Ran shell syntax checks:
• bash -n lib/teams.sh
• bash -n lib/agents.sh
• bash -n tinyclaw.sh
• Ran local behavior tests using temporary TINYCLAW_HOME + test settings.json:
• team add-agent successfully adds member
• team remove-agent handles leader removal and leader reassignment
• agent remove warns about team impact and removes team membership together
• team is deleted automatically when it becomes empty

Checklist

• I have tested these changes locally
• My changes don't introduce new warnings or errors
• I have updated documentation if needed

[jlia0]

thank you!!