feat: establish Phase 1 workflow domain contracts

[eshulman2]

Summary

• define strict, immutable, versioned Forge domain contracts
• convert source-control NormalizedEvent into deterministic webhook/poller Observation objects
• project broad checkpoint/provider facts into a scoped implementation-input request
• run implementation resolution as a provider- and LangGraph-independent station
• validate station outcomes through an allowlisted reducer owning four checkpoint fields
• provide a serialized local station runner and architecture/conformance tests
• document Phase 1 delivery, implications, and coupling measurements

Stack

This PR is intentionally stacked on PR #318 and remains separate from PRs #320, #317 and #318. All downstream branches were rebased after the latest PR #320 update.

Compatibility

• existing BaseState checkpoints remain valid
• graph topology and routes are unchanged
• the existing resolve_implementation_input(state, jira) call shape is preserved
• effects remain inline until Phase 3

Verification

• make lint
• uv run pytest -q tests/unit — 2396 passed
• uv run pytest tests/workflow tests/contracts tests/flows -q --strict-markers — 412 passed