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/BUG_REPORT.md

@@ -0,0 +1,11 @@
+# Bug Report: Pre-existing Fixes
+
+## Bugs Found: 0
+
+All fixes verified:
+1. `_infer_state_from_artifacts`: `spec_only → research` confirmed by test
+2. `_infer_state_from_artifacts`: `implementation_only → bug_find` confirmed by test
+3. `cmd_validate_folder`: empty `.state` → error, confirmed by test
+4. `cmd_validate_folder`: whitespace `.state` → error, confirmed by test
+5. `upgrade.sh`: `|| true` removed, python3 check added, git-dir detection fixed
+6. All 206 tests pass