Add deterministic GitHub replay harness

[2002yy]

What changed

• add a schema-v1 immutable GitHub PR replay manifest and loader
• aggregate deterministic symbol-mapping, CI-association, corpus, and provider metrics
• migrate PR feat: add version-aware GitHub change impact #28/refactor: enforce one task contract per turn #30 curated labels into one manifest source of truth
• add CLI, regression coverage, and roadmap/status audit updates

Why

G10-C3a needs an explicit replay baseline before expanding repository execution capabilities. Curated seeds are reported separately from real provider replays, preventing small unit fixtures from being presented as representative quality evidence.

Validation

• focused replay/evaluation: 7 passed
• GitHub suite: 97 passed
• full pytest: 781 passed
• Ruff: passed
• targeted mypy: passed
• package structure and secret scan: passed (1018 files)