feat: uwf thread read — human-readable markdown with pagination

- Outputs markdown directly (not JSON/YAML)
- --quota <chars>: character budget, loads steps backward until exceeded (default 4000)
- --before <step-hash>: load steps before this hash (exclusive), omits start
- --start: force include start section even with --before
- --detail: expand detail CAS node content for each step
- Skip hint with uwf thread read command for pagination
- Reuses walkChain/collectOrderedSteps/expandOutput

Closes #349

.gitignore

@@ -10,3 +10,4 @@ xiaoju/
 solve-issue-entry.ts
 packages/workflow-template-develop/develop.esm.js
 .DS_Store
+*.py