feat: add Gemini CLI as a provider

[williamwarlick]

Summary

Add Google Gemini CLI (@google/gemini-cli) as a third provider alongside Claude and Codex. This lets nightshift utilize unused Gemini token budgets for overnight task execution.

Motivation

Gemini CLI supports headless execution (gemini -p <prompt> -y --output-format json), model selection (-m), and stores session data with token usage locally at ~/.gemini/tmp/. This maps cleanly onto nightshift's two-layer provider architecture (provider for budget tracking, agent for task execution).

Proposed Changes

Provider layer (internal/providers/gemini.go):

• Parse Gemini CLI session files to calculate daily/weekly token usage
• Implement GetUsedPercent, GetTodayUsage, GetWeeklyUsage following the existing provider pattern

Agent layer (internal/agents/gemini.go):

• Spawn gemini -p <prompt> -m <model> -y --output-format json for headless task execution
• Support configurable model (e.g., gemini-3-pro-preview)
• Auto-approve flag support via -y

Config (internal/config/config.go):

• Add gemini to ProvidersConfig with enabled, data_path, and model fields

Budget (internal/budget/budget.go):

• Integrate GeminiUsageProvider into the budget manager

Commands:

• Wire gemini into run, daemon, budget, preview, doctor, task, and snapshot subcommands
• Add "gemini" to provider preference and candidate selection

Testing

• 10 provider tests (session parsing, token summing, usage percent, edge cases)
• 8 agent tests (defaults, options, execution, timeout, file context, errors)

Notes

• Snapshot collector integration left as follow-up
• Requires gemini CLI binary in PATH (installable via npm i -g @google/gemini-cli or homebrew)

[marcus]

Hey @williamwarlick! Starling here (AI assistant on the project). 👋

Gemini CLI support is a natural ask — especially given it's now available locally. I see you've also opened PR #25 with an implementation already, nice! Flagging both for Marcus to review. ✦