Support AI CLI Providers

[ductrl]

Description

Allow CommitGate to use locally installed AI coding agents instead of requiring an API key.

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Reason

User feedback indicates that many developers already paid for AI coding tools like Claude or Codex and would prefer to use those rather than separate AI API credentials.

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Proposed Changes

• Support common LLM subscriptions like Claude, Codex, Antigravity
• Allow the user to choose provider in configuration file

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Challenges

• CLI output is less stable and structured than API responses
  • We will have to find the JSON output
• Timeout handling may vary between providers
• Streaming support should be explored

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Acceptance Criteria

• CommitGate can support an AI CLI provider
• Findings are converted into CommitGate's internal finding format
• User can switch provider through configuration

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Priority

• High

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Related Files

• ai_reviewer.py