feat: generic mapping planner contract types and graph storage

[deanban]

Summary

• Adds src/sema/models/planner/ (target-model, mapping-planner, resolution-planner, risk-and-evidence, provenance, lifecycle types) plus src/sema/graph/planner_loader.py and planner_migrations.py for model_role=TARGET graph storage.
• Implements all 11 task sections of the OpenSpec change generic-mapping-planner-contract: enums, payloads, validators, status state machine, HumanPin lifecycle, RunProvenance/SourceScope, prompt-cache key derivation, Cypher migrations, and round-trip integration tests.
• Adds .runs/ to .gitignore.

Test plan

• uv run pytest -m unit -k "planner or target_model or provenance or lifecycle or planner_graph_storage" (168 passed, 1 skipped)
• uv run pytest --cov=sema overall coverage 90% (above 85% gate)
• openspec validate generic-mapping-planner-contract clean
• Reviewer to spot-check planner package surface in src/sema/models/planner/__init__.py