ci: document and gate product exit codes in README tables

[TMHSDigital]

Summary

• 3a (50a26c7): headless-batch-script-template README now has a CONTRIBUTING-shaped exit table for 0, argparse/no mesh 2, modifier 3, glTF 4. Script unchanged. extension-addon-template has no sys.exit and needs nothing. ai-asset-pipeline-template already had a table.
• 3b (b1c1268): tests/check_exit_code_readme.py, wired blocking in validate.yml. AST only. No exemption list.
• Closes ci: report-mode check that every nonzero exit is documented in its own script README #138 (blocking from day one, not report-mode).

Detection: integer return N plus literal sys.exit(N) in examples/, templates/, showcase/. sys.exit(main()) / sys.exit(exit_code) after exit_code = main() are harness pass-through. sys.exit(1) in except is FATAL. README check is entry-point files only (if __name__ == "__main__"). Helpers such as gallery_framing.py are still scanned for unanalyzable sys.exit.

No non-literal product exits in the tree. No skip file.

Gate-failure proof (verbatim)

===== 1-undocumented-return exit=1 =====
ERROR: examples/bmesh-gear/bmesh_gear.py:218: return/exit 99 is not in the examples/bmesh-gear/README.md exit table

===== 2-unanalyzable-exit exit=1 =====
ERROR: examples/bmesh-gear/bmesh_gear.py:223: unanalyzable exit code: sys.exit(undocumented)

===== 3-fatal-guard-clean exit=0 =====
exit-code README checks passed.

===== 4-kit-exit-10-undocumented exit=1 =====
ERROR: examples/modular-kit-snap/modular_kit_snap.py:629: return/exit 10 is not in the examples/modular-kit-snap/README.md exit table

===== 5-clean-tree exit=0 =====
exit-code README checks passed.

Canaries were applied, observed, and deleted. Working tree after #5 matches HEAD. Canary 3 is the same checker run against the tree that still has the normal __main__ sys.exit(1) FATAL wrappers (e.g. bmesh-gear).

Test plan

• Validate (new checker step green)
• Blender 5.2 LTS smoke
• Blender 4.5 LTS smoke
• Socket Security
• Drift check

No example behavior change — needs-5.1 not applied.

Live run: canaries above. Inspection: template script.py untouched; validate.yml step next to check_import_export_rules.py.