feat: add Gemini CLI support for spawn backends

[he-yufeng]

Summary

Adds first-class support for Google's Gemini CLI (gemini) as a spawn target, alongside Claude, Codex, and Nanobot. Closes #23.

Changes

Both backends (tmux_backend.py, subprocess_backend.py):

• _is_gemini_command() — detects gemini binary (with full path support)
• Permission skipping via --yolo flag (Gemini's equivalent of Claude's --dangerously-skip-permissions)
• Prompt injection via -p flag

Tmux backend additionally:

• Workspace trust confirmation for Gemini's "Trust folder" dialog
• _is_interactive_cli() updated to include Gemini
• Generic send-keys fallback skips Gemini (prompt already baked into command)

Tests: 3 new tests covering tmux spawn, subprocess spawn, and workspace trust detection. All 152 tests pass.

Usage

# team config
[agents.researcher]
command = ["gemini"]

clawteam spawn researcher --skip-permissions --prompt "analyze this repo"
# → gemini --yolo -p 'analyze this repo'

Test plan

• pytest tests/test_spawn_backends.py — 14/14 pass
• pytest (full suite) — 152/152 pass
• ruff check — clean