Add phase-2 feature grouping algorithm (#133)

[Dimwiddle]

Description

Implement deterministic Phase 2 discovery grouping for DiscoveredItem inputs.

This adds:

• ordered grouping heuristics (file-path proximity, API route prefix, name-prefix fallback)
• git-commit cross-reference assignment via GitCommitMeta.file_prefixes
• DraftFeature/DraftScenario construction with abbreviation expansion and confidence scoring
• acceptance-focused unit coverage for grouping behavior
• feature spec updates documenting grouping scenarios and criteria

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation-only

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Testing

• make lint-fix -- passing
• make lint -- passing
• make test -- passing (503 tests)
• make pre-commit -- passing

Related Issues

Closes #133