🚀 ci: auto-publish beta to TestPyPI on develop merges

[andycreed0x]

Purpose

Automate beta releases: every merge to develop now bumps the beta version and publishes it to TestPyPI, removing the manual src/aqua/__init__.py edit. main no longer publishes; PyPI stays release-only (published via a GitHub Release).

Description

develop becomes the beta channel — each push auto-increments the PEP 440 beta segment and ships a distinct bN to TestPyPI. The final (non-beta) release is cut from main, which no longer triggers this workflow: set __init__.py to the clean version there and publish a GitHub Release to PyPI.

The auto-bump commit is pushed back to develop with the default GITHUB_TOKEN. develop requires the Run tests status check, so the first merge is effectively a live test of whether that push is allowed — if it is rejected, the fix is in develop's protection settings (let GitHub Actions bypass the rule), not a new personal token.

Main Changes

• 🚀 Publish to TestPyPI on every develop push; drop main from the workflow trigger
• ✨ Add scripts/bump_beta.py — increments the beta segment (X.Y.ZbN → b(N+1), or X.Y.Z → b1) and fails loudly on non-beta version strings (no silent fallback)
• 🔧 Auto-bump step commits the new version back to develop as github-actions[bot] with [skip ci]; per-ref concurrency serializes rapid merges
• 🚚 Move PUBLISHING.md → docs/PUBLISHING.md and rewrite it for the new CI flow
• 📝 Reference docs/ in the AGENTS.md layout table

⚠️ Breaking Changes

• Pushing to main no longer publishes to TestPyPI.
• Cutting a final release changes: edit __init__.py to the clean version on main, then publish a GitHub Release → PyPI. Any push to develop always appends/bumps bN, so a clean version cannot land on develop.

Checklist

• No hardcoded values (they should go in constants.py, .env, or our database)
• Added/updated tests (if necessary)
• Added/updated relevant documentation (if necessary)