refactor(function-agents): route startchat through facade

[limityan]

Summary

• Route Startchat work-state analysis through FunctionAgentRuntimeFacade using core-owned Git and AI adapters.
• Preserve legacy Startchat behavior for no-HEAD git diff fallback, non-Git empty state, analyze_git=false time-info, and post-analysis analyzed_at assignment.
• Extend boundary checks and docs so Startchat orchestration stays facade-backed while Git/AI service calls, prompt templates, JSON extraction, and error mapping remain core-owned.

Functional Boundary

This PR is intended to be behavior-preserving. It does not move Git service execution, AI client selection, prompt templates, JSON extraction, or error mapping out of core. It only changes the Startchat work-state product path to use the already-defined product-domain facade with the same core-owned adapters underneath.

Validation

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