Tasks: a linked PR missing one arming label is never polled

Task list

[x] A1: red tests. Nine unit tests in test_github_ops.py and the Gherkin integration test, all failing against the pre-fix code. Req: R1–R4 · Test: T1–T4.
[x] A2: one reader of the list. github_ops.auto_execute_labels, with the dispatcher's default and normalisation. Req: R1 · Test: T1. Deps: A1.
[x] A3: arm a linked PR. _arm and github_ops.link_pull_request; create_pull_request and discover_pull_requests call them. Req: R1–R4 · Test: T1–T3. Deps: A2.
[x] A4: every link-pr entry point. The facade (service route and MCP tool), the SDK client and the CLI's in-process fallback call github_ops.link_pull_request; work_item.pr_labelled joins the event catalogue. Req: R2 · Test: T4, T5. Deps: A3.
[x] B1: skill text. reference/automation.md, the pull-requests.md template, /the-loop:work-on, /the-loop:execute-tasks. Req: R5. Deps: A3.
[x] C1: docs and evidence. The CLI reference (sessions, pr), the capability docs cli.md and webhook-triggers.md, and the evidence files. Deps: A4, B1.