Fill the PR template and label by: ai on generated PRs

[bracyw]

Changes

CI's propose PRs and new-workspace.sh's PRs now fill the PR template's sections (Changes with the workspace commits, Testing, Checklist) and are labelled by: ai when opened. Sync never touches labels afterwards, so a changed written-by label stays; new-workspace.sh takes --by ai-assisted|human.

Testing

• e2e: a propose PR has the template's sections and the by: ai label
• e2e: new-workspace's PR is labelled by: ai by default, by: ai-assisted with --by, and a bad --by is rejected

Notes

• Created the missing by: ai and by: human labels on this repo (copied from Argos); gh pr create fails on a label that doesn't exist.
• The bodies are written out in the scripts, so a template change needs them updated too (noted in CLAUDE.md).

Checklist

• ci/check.sh . and bash tests/e2e.sh pass
• Version bumped if a .delphi/manifest.yml file changed (none changed)
• No merge conflicts
• Request reviewers & ping on Slack