refactor(function-agents): wire commit facade path

[limityan]

Summary

• Route Git commit-message generation through FunctionAgentRuntimeFacade while keeping concrete Git and AI execution in core-owned adapters.
• Preserve path fidelity across the function-agent Git port by passing PathBuf instead of lossy string paths.
• Split Startchat time facts into StartchatTimeSnapshot so future work-state wiring can preserve the existing analyze_git=false time-info behavior.
• Extend boundary checks and docs to lock the new facade seam and keep Startchat work-state on the existing core path until equivalence is proven.

Functional Boundary

This PR is intended to be behavior-preserving. It does not move Git service execution, AI client selection, prompt templates, JSON extraction, error mapping, or Startchat work-state execution out of core. The only product call-path change is the Git commit-message orchestration now flowing through the product-domain facade with the same core adapters underneath.

Validation

• cargo test -p bitfun-product-domains --no-default-features
• cargo test -p bitfun-product-domains --features product-full
• cargo check -p bitfun-core --features product-full
• cargo test -p bitfun-core function_agents:: -- --nocapture
• node scripts/check-core-boundaries.mjs
• cargo tree -p bitfun-product-domains --no-default-features --depth 1 --edges features
• rustfmt --edition 2021 --check src\crates\core\src\function_agents\git-func-agent\commit_generator.rs src\crates\core\src\function_agents\port_adapters.rs src\crates\product-domains\src\function_agents\ports.rs src\crates\product-domains\tests\function_agent_contracts.rs
• git diff gcwing/main...HEAD --check