feat(backends): add gemini-cli backend to standalone CLI

[devswha]

Follow-up to #69 (v2 phase). v1 (codex-cli) shipped via PR #70.

Goal

Add a gemini-cli backend that spawns gemini -p '' --output-format text with the patina prompt on stdin. Together with claude-cli, completes the no-API-key triad (codex / claude / gemini).

Sketch

• src/backends/gemini-cli.js mirroring codex-cli.js shape
• isAvailable() checks gemini --version
• isAuthenticated() checks the gemini CLI's credential file location (verify in CLI docs)
• invoke() spawns gemini -p '' --output-format text, pipes prompt to stdin
• Register in src/backends/index.js REGISTRY
• Heuristic: --model gemini* → gemini-cli
• Per security: require explicit opt-in or sandboxing for codex-cli backend #88 hardening: no silent auto-fallback; require explicit --backend gemini-cli (or --model gemini*)

Edge cases

• Output post-processing if gemini wraps with markdown code fences
• Timeout: gemini CLI start latency higher than HTTP

Acceptance

• patina --backend gemini-cli input.txt works when gemini is installed and authenticated
• patina --model gemini-1.5-pro input.txt routes to gemini-cli
• patina auth status reports availability + auth state
• No silent auto-fallback (per security: require explicit opt-in or sandboxing for codex-cli backend #88 policy)
• Tests: backend mock + selection routing
• Docs: README CLI section lists gemini-cli alongside codex-cli/claude-cli

Out of scope

• Mixed-backend MAX mode (tracked separately once both v2 backends land)