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/VERDICT.md

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+# Verdict: pre-existing-fixes
+
+## Status: PASS
+**Completion Date**: 2026-06-15
+
+## Summary
+Fixed 4 pre-existing issues:
+1. `_infer_state_from_artifacts` heuristic: SPEC-only now correctly returns `research` instead of `bug_find`; IMPLEMENTATION-only returns `bug_find` (implement done → next phase)
+2. `cmd_validate_folder`: corrupted `.state` files now error instead of silently falling back to artifact inference
+3. `upgrade.sh`: removed `|| true` so upgrade failures are visible; added python3 availability check; uses `git rev-parse --git-dir` for hooks directory (supports worktrees)
+4. 6 new tests covering all fixes
+
+## Findings
+- All 206 tests pass
+- No regressions introduced
+- Heuristic fix aligns with existing `TestStateMachineAlignment` tests
+
+## Score
++10