Change how the workspace's pull request bodies are laid out (owner or admin)

Replaces formatting (off leaves every template out, so the fields land on the platform's default layout), template_mode (repository_first: the repository's template, else default_template; override: default_template always, the repository's is not read), default_template (markdown up to 16 KiB, or null for the platform's default) and session_link (id leaves the console's address out of the pull request tail and the Agent-Session trailer). revision is pull_request_revision as last read; a stale one is 409 REVISION_CONFLICT and nothing changes. It is separate from the ceiling's revision. A template the platform will not lay text around is 400 with reason_code template_too_large, template_not_utf8 or template_unsupported. A change reaches pull requests opened or updated afterwards and commits made after a worker's next claim; text already on GitHub is not rewritten. Writes a set_github_pull_request_settings receipt with the values before and after (a template as its size and hash, never its text); receipt_id is null when nothing changed. API keys are refused.

Auth: cookie session only; no scope. Idempotency-Key: ignored.