chore: improve tooling quality and docs

- close_issue.sh: add set -euo pipefail, argument validation, confirmation output
- mark_issue.sh: track previously untracked claim/release script
- create_pr.sh: remove hardcoded one-off (use create_pr.py instead)
- README.md: reflect current toolset with usage examples
- .gitignore: ignore venv/ and __pycache__/

.gitignore

@@ -0,0 +1,3 @@
+venv/
+__pycache__/
+*.pyc