Tasks: `the-loop pr ready`

Task list

[x] A1: red tests. Client, core, CLI and integration tests for pr ready, failing against the code before the change. Req: R1, R2 · Test: T1–T5.
[x] A2: the client method. GitHubClient.mark_pull_ready and the fake's counterpart. Req: R1.1, R1.5 · Test: T1. Deps: A1.
[x] A3: the core verb. github_ops.mark_ready and the work_item.pr_ready event. Req: R1, R2.5 · Test: T2, T3. Deps: A2.
[x] A4: every seam. CLI subcommand, route and body, API facade, manager facade, MCP tool, OpenAPI contract. Req: R2 · Test: T4–T6. Deps: A3.
[x] B1: skill text. SKILL.md, reference/automation.md, /the-loop:work-on. Req: R3. Deps: A3.
[x] C1: docs and evidence. CLI reference (pr, command index), capability docs (cli.md, control-plane.md), the evidence files. Deps: A4, B1.