fix(hooks): emit PostToolUse additional context

[KumamuKuma]

Summary

• extract the inline PostToolUse auto-update logic into a portable Node handler
• emit hookSpecificOutput.additionalContext so the instruction reaches model context
• preserve the existing Git-operation, config, graph, and .ua fallback guards
• add regression coverage for successful triggers and silent failure paths

Context

Follow-up to #598. That PR fixed the hook input by reading the PostToolUse payload from stdin, but the handler still wrote a plain-text message to stdout. PostToolUse ignores plain stdout, so the commit-triggered auto-update instruction was never added to model context.

Validation

• pnpm lint
• pnpm --filter @understand-anything/core build
• pnpm --filter @understand-anything/skill build
• pnpm --filter understand-anything-viewer build
• pnpm --filter @understand-anything/core test
• pnpm test (new hook tests: 14/14)
• Python helper suite: 95 tests, 1 skipped
• git diff --check origin/main...HEAD