Improve TaskBound scope inference for PR reviews

[Conalh]

Scope

• src/
• test/
• action.yml
• README.md
• .github/workflows/taskbound.yml

Summary

• Add PR title/body task sourcing so the GitHub Action can use pull request context without requiring a manual task input.
• Add optional --scope-llm <model> scope extraction with heuristic fallback when offline or unauthenticated.
• Add .taskbound.yml severity overrides and stable finding category fields for future signal package extraction.
• Reduce PR-body scope noise by ignoring validation sections and accepting explicit repo-root directory scope.
• Avoid treating repository test/fixtures/ package files as live install surfaces.

Validation

• npm run build
• npm test (18/18 passing)
• git diff --check
• Local smoke: node dist/index.js review --github-event test/fixtures/github/pull_request_event_with_validation.json --repo . --base main --head HEAD --format markdown
• Local smoke: node dist/index.js review --task "Fix header CSS styling" --scope-llm gpt-test-scope --old test/fixtures/scope-creep/old --new test/fixtures/scope-creep/new --format json