refactor(agent): extract hasTTY env checks to NonInteractiveDetector

[mvanhorn]

Summary

Closes #601

Replaces the hardcoded environment variable checks in hasTTY() with a new optional NonInteractiveDetector interface that agents can implement. This makes the non-interactive detection extensible - new agents can declare their env vars without touching the strategy package.

Before

// manual_commit_hooks.go - hardcoded checks
if os.Getenv("GEMINI_CLI") != "" { return false }
if os.Getenv("COPILOT_CLI") != "" { return false }
if os.Getenv("GIT_TERMINAL_PROMPT") == "0" { return false }

After

// agent.go - optional interface
type NonInteractiveDetector interface {
    Agent
    NonInteractiveEnvVar() (name string, value string)
}

// manual_commit_hooks.go - single call
if agent.IsNonInteractiveEnv() { return false }

The (name, value) tuple return handles both patterns:

• "Any non-empty value" (Gemini, Copilot) - return ("GEMINI_CLI", "")
• "Exact value match" (Factory AI Droid) - return ("GIT_TERMINAL_PROMPT", "0")

Changes

• agent/agent.go - Added NonInteractiveDetector interface
• agent/registry.go - Added IsNonInteractiveEnv() helper that iterates registered agents
• agent/registry_test.go - 5 subtests covering empty registry, unset var, any-value match, exact-value match, and exact-value mismatch
• agent/geminicli/gemini.go - Implements NonInteractiveEnvVar() -> ("GEMINI_CLI", "")
• agent/copilotcli/copilotcli.go - Implements NonInteractiveEnvVar() -> ("COPILOT_CLI", "")
• agent/factoryaidroid/factoryaidroid.go - Implements NonInteractiveEnvVar() -> ("GIT_TERMINAL_PROMPT", "0")
• strategy/manual_commit_hooks.go - Replaced 3 hardcoded checks with single agent.IsNonInteractiveEnv() call

Testing

• 5 new unit tests for IsNonInteractiveEnv() covering all edge cases
• Uses mock agent struct with test-only registration/cleanup
• All existing tests continue to pass

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Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com