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/readme-upgrade-docs/ADVERSARIAL_BUG_REPORT.md

@@ -0,0 +1,10 @@
+# Adversarial Bug Report: README and Upgrade Docs
+
+## Deep Bugs Found: 0
+
+Documentation-only changes. No code paths altered.
+
+## Pre-existing Issues (not in scope but noted)
+1. `upgrade.sh` silently swallows `status.py --upgrade` failures
+2. Pre-commit hook doesn't handle bare repos or worktrees
+3. `_infer_state_from_artifacts` heuristic bug (SPEC.md-only → bug_find instead of research)