Turn one GitHub repository on or off for the agent

One toggle, one change, no release: a new session reads it at once, and turning a repository off reaches running sessions too. repository is a ref from GET …/github (selected_repositories, options.repositories) to turn a repository the catalog holds on or off, or installation_id with github_repository_id from GET …/github/repositories to turn on one the catalog may not hold yet; a repository is turned off by its ref only. The catalog row (added, or moved to a new name after a rename on GitHub), the settings, the receipts and the author App pin are written in one transaction, so a refusal or a failure anywhere leaves none of them. The change is applied to the row as it stands, so toggles of other repositories are kept and the last one of a repository stays. expected_revision is optional: left out, nothing is compared; given (null when there was no row) and different, the answer is 409 REVISION_CONFLICT with details.revision, and the caller reads GET …/github again. Turning off the repository start_repository_id names clears start_repository_id. An agent with no settings row starts from its active release's GitHub pins, read under the agent's lock; with no active release, or pins this image cannot read, that is 409 REVISION_CONFLICT with reason_code: settings_seed_unavailable. 400 reason_code: invalid_ref, repository_not_found, repository_not_allowed (the workspace's repository ceiling; a repository picked from an installation also needs a ceiling that runs on the App's repositories), app_required, name_held (another row has that name), no_author_app or not_installed (as POST …/github/repositories). Writes a set_agent_github receipt with result.change = "repository" when the settings changed; receipt_id is null when the repository already was in that state. Agent managers with a web session only; API keys are refused.

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