Merge main into dev to unblock test->main promotion

Reconciles main's PR #4 commit (cb39a3a 'Simplify CI/CD: remove skeleton
workflows, add initial deployment guide') back into the dev->test->main
flow. PR #4 was merged directly to main, leaving dev/test missing main's
history.

Conflict resolution takes dev's tree in all cases:
- Modify/delete: scripts/branch_env.py and validate-branch-env.yml were
  renamed in PR #5 to workspace_swap.py and check-pr-ready.yml -- keep
  the renames.
- tests/test_branch_env.py renamed to test_workspace_swap.py -- keep the
  rename.
- README.md, fabric-development-process.md, copilot-instructions.md:
  PR #5 rewrote these end-to-end with the new tool names; main's older
  PR #4 wording is superseded.
- .gitignore: PR #5's superset already covers main's *.pyc.
- notebook-content.py: kept dev's Fabric-synced version.

Net result: dev's working tree is unchanged; main's commit is now part
of dev's history so future dev->test->main promotions fast-forward.

Author: michaeldeongreen