refactor(core): split project workflow planning

[phernandez]

Summary

• move project-index workflow plans and persisted checkpoint models into a focused models module
• move start/progress/result planning and stale recovery into separate implementation modules
• preserve the existing project_index_workflow import surface as a 41-line compatibility facade

Related issue

Part of #1108

Testing

• just fast-check
• uv run pytest -q tests/indexing/test_project_index_workflow.py (15 passed)
• just fast-test (13 passed, 1 skipped)
• just package-check
• git diff --check

Risk

Low. This is a mechanical module split with the public import surface and persisted metadata shapes unchanged. Existing tests assert exact workflow metadata, event payloads, progress, idempotent batch recording, and stale recovery behavior.