Fix pre-existing issues and update README for v2.0

- Fix _infer_state_from_artifacts: SPEC-only maps to research (was bug_find)
- Fix cmd_validate_folder: corrupted .state files now error instead of silent fallback
- Fix upgrade.sh: remove || true, add python3 check, use git rev-parse --git-dir
- Update README.md with --project flag, --can-edit modes, --upgrade, enforcement layers
- Add 6 new tests for inference and validation fixes
- Complete hook-install-process and readme-upgrade-docs tasks

tasks/pre-existing-fixes/DOC_REVIEW.md

@@ -0,0 +1,13 @@
+# Doc Review: Pre-existing Fixes
+
+## Review Summary
+
+All documentation and code comments are accurate.
+
+## Checklist
+- [x] `upgrade.sh` comments and error messages are clear
+- [x] `_infer_state_from_artifacts` docstring unchanged (function has no docstring — relies on code clarity)
+- [x] `cmd_validate_folder` error message for corrupted state is clear and actionable
+- [x] No stale references to removed behavior (no || true, no `.git/hooks` assumption)
+- [x] Tests are well-documented with descriptive names
+- [x] IMPLEMENTATION.md accurately describes all changes