[codex] Fix force overwrite sidecar contract

[DavidJBianco]

Summary

• Make every successful generation emit the complete sidecar set: data/, GROUND_TRUTH.md, and OBSERVATION_MANIFEST.json.
• Keep the eforge generate --force transactional swap strict, now requiring staged data, ground truth, and observation manifest before replacing old output.
• Clean up empty baseline-only ground truth IOC rendering and update docs/skill references to reflect that ground truth is generated for every run.

Why

Prior fixes protected the swap by requiring staged GROUND_TRUTH.md, but baseline-only generations could still skip ground truth entirely. That made successful generation fail during overwrite. The root contract is now explicit: successful generation always produces matched data and sidecars, so the swap can stay strict instead of accepting partial output.

Validation

• uv run pytest tests/unit/test_engine.py tests/unit/test_cli.py tests/unit/test_ground_truth.py tests/unit/test_observation_manifest.py --no-cov: 112 passed
• uv run eforge validate-config: passed with 0 errors, 0 warnings
• uv run ruff check .: passed
• uv run ruff format --check .: passed
• uv run pytest -v: 3051 passed, 15 skipped

Slow tests were not run.