Add safe GitHub Releases and collaboration guides

[pattyboy813]

Summary

• add a tag-only release workflow that validates strict annotated SemVer tags on main, synchronized Python/Worker versions, and the complete CI suite before creating generated GitHub release notes
• keep release publishing idempotent and bind it to the exact validated tag object, with contents: write limited to the publish job
• document learner-safe Git, release, Issue, PR/review, and native stacked PR workflows
• add focused contributing guidance, Issue forms, a PR template, and generated-notes categories

This is a normal independent PR against main; it is not part of a stack. Creating a GitHub Release does not deploy the Worker or restart/deploy the Discord bot.

Checks

• strict validator valid case and rejected tag/version/history cases
• Python compile and Ruff
• pytest: 29 passed
• Worker typecheck and Vitest: 90 passed across 8 files
• workflow/Issue YAML parsed and release shell syntax checked
• final staged diff reviewed for permissions, tag identity, quoting, and secret/deployment risks

Risk and rollout

No product behavior changes. After merge, the first release remains an explicit annotated-tag action by a maintainer; production deployment remains separate and manual.