feat: implement gr2 team subcommand

[laynepenney]

Summary

• Implements gr2 team add/list/remove subcommand for managing agent workspaces
• Follows the existing repo/unit pattern: TeamCommands enum in args.rs, dispatch logic in dispatch.rs
• Creates agents/<name>/agent.toml with name and kind = "agent-workspace" fields
• Fixes 9 pre-existing TDD test failures on main (45/45 cli_tests now pass)

Details

The tests were merged as TDD specs but the implementation was never wired up, violating the "main never has failures" policy. The implementation is 88 lines across two files, closely following the existing unit and repo subcommand patterns.

Premium boundary: core OSS (gr2 workspace orchestration CLI).

Test plan

• All 9 test_gr2_team_* tests pass
• Full cli_tests suite passes (45/45, up from 36/45)
• cargo clippy -- -D warnings clean
• cargo fmt --all --check clean
• No regressions in other test suites (remaining 5 failures are pre-existing grip#606 TDD specs)

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