refactor: begin Phase 2 event interpretation boundary

[eshulman2]

Summary

• introduce an infrastructure-free ingress adapter contract and registry
• move Jira ticket-type/observation interpretation and source-control observation/PR-correlation evidence out of OrchestratorWorker
• make worker source registration data-driven
• derive stable, versioned workflow commands for approval, rejection, retry, questions, checks, reviews and merges in shadow mode
• make unmatched/wrong-stage signals explicitly inspectable as ignored decisions
• document the remaining Phase 2 delivery slices and compatibility rules

Stack

• Depends on feat: establish Phase 1 workflow domain contracts #324
• Phase 2 work continues here: exceptional handlers, authoritative command application, durable decision evidence and conformance remain

Verification

• uv run ruff check src/forge/orchestrator/worker.py src/forge/orchestrator/event_adapters tests/unit/orchestrator/event_adapters
• uv run pytest -q tests/unit/orchestrator/event_adapters tests/unit/orchestrator/test_worker.py (103 passed)