Document and check the minimum gh version for GitHub App workers

[danny-avila]

The worker injects GH_TOKEN so agents can use gh, but does not check the installed gh. Distribution packages lag far behind: Ubuntu's apt gh is 2.45.0. In that version, gh pr edit always requests the retired Projects (classic) projectCards field (pkg/cmd/pr/edit/edit.go), so every edit fails:

GraphQL: Projects (classic) is being deprecated in favor of the new Projects experience ... (repository.pullRequest.projectCards)

Current gh uses feature detection (cli/cli#10942, with the remaining case fixed in v2.82.1), and the maintainers direct users to the official apt repository rather than distro packages (cli/cli#11983, cli/cli#13069). On one trusted-VM deployment, agents hit this 54 times in 9 days and fell back to gh api -X PATCH.

Proposal

• README / worker runbook: require gh >= 2.82.1 from GitHub's official apt repository when GitHub App auth is enabled, with the install commands.
• At startup (or in a doctor command), when GitHub App auth is configured and gh is on PATH, log a warning if gh --version is older than the minimum.

Acceptance

• The runbook states the minimum version and install source.
• A worker with an older gh logs one clear warning at startup naming the installed and required versions.