feat(ci): clean up submission PR titles and automate image localization

[pfeerick]

Summary

• Strip the issue-template prefix (and drop the — separator) from the auto-generated submission PR title and commit message. [Lua App Submission] EdgeTX Flight Log now produces feat(scripts): Add EdgeTX Flight Log instead of feat(scripts): Add script — [Lua App Submission] EdgeTX Flight Log.
• Add localize-images.yml: applying the new localize-images label to a submission PR now runs tools/download_external_images.py and pushes the result as a commit, replacing the manual "run this locally" checklist step in the draft PR body. The label already exists on the repo (#5319E7).
• Add CONTRIBUTING.md documenting the full issue → label → automated draft PR → localize-images → merge → gh-pages pipeline.
• Bump pinned GitHub Action versions across all workflow files: actions/checkout v6→v7, astral-sh/setup-uv v8.1.0→v9.0.0, actions/upload-pages-artifact v3→v5, actions/deploy-pages v4→v5. (actions/github-script and actions/upload-artifact were already pinned to their current latest majors.)

Test plan

• Traced the title/commit-message regex by hand against [Lua App Submission] EdgeTX Flight Log and [Lua App Update] EdgeTX Flight Log — both produce the expected clean strings.
• uv run tools/validate_scripts.py --scripts-json scripts.json
• uv run tools/sync_issue_template_options.py --check
• uv run tools/test_sync_issue_template_options.py
• uv run tools/download_external_images.py --dry-run (confirms the script itself is untouched)
• All workflow YAML re-parsed with PyYAML after edits
• End-to-end: open a test submission issue, apply add-to-gallery, confirm the draft PR title/commit are clean, then apply localize-images and confirm the auto-commit lands

🤖 Generated with Claude Code