feat: add Phase 7 execution read models

[eshulman2]

Summary

• add a versioned operator read model over checkpoint position, pinned process definition, station attempts and durable effects
• explain running, waiting, blocked, failed and completed states without Jira-label or log inference
• expose permitted commands and manifest-derived next transitions
• report observation availability/freshness and external-state conflicts
• expose migration eligibility/incompatibilities when revision analysis is available
• retain canonical declarative definitions with checkpoints so inspection uses the pinned revision
• retain compact station-attempt evidence without storing complete request/output payloads
• add GET /api/v1/workflows/{ticket_key}/execution

Stack

• Depends on feat: begin Phase 5 explicit process definitions #328; Phase 6 is intentionally not part of this branch
• Implements Phase 7 execution projection, pinned-definition visibility and operator API slices
• Durable command/observation timelines and Org Pulse integration remain follow-up slices

Verification

• uv run ruff check src/ ...
• focused mypy checks for read models, API loader and station audit helper
• uv run pytest tests/unit/ tests/workflow tests/contracts tests/flows -q --strict-markers — 2,842 passed