Tasks: resolve a PR's linked issues first (issue-93)

Phase 3 of 3. Derived from the locked design.md. TDD (tdd.mode: standard): the failing test comes first for every behavioural task.

DAG

flowchart LR
    T1[T1 router tests] --> T2[T2 _pr_entity + keyword grammar]
    T2 --> T3[T3 linked_issue_numbers]
    T3 --> T4[T4 extract_work_items ordering]
    T4 --> T5[T5 webhook integration test]
    T4 --> T6[T6 gh client tests]
    T6 --> T7[T7 closingIssuesReferences + downgrade]
    T7 --> T8[T8 provider payload injection]
    T8 --> T9[T9 poll integration test]
    T5 --> T10[T10 lint/typecheck/tests]
    T9 --> T10
    T10 --> T11[T11 capability docs + decision]
    T11 --> T12[T12 PR + briefing]

Tasks

• [x] T1 — Failing router unit tests. In cli/tests/test_routing.py: PR conversation comment (issue_comment with issue.pull_request) → [issue, PR]; closingIssuesReferences honoured; OWNER/REPO#N, GH-N, Fixes: #N, issue URL resolve; cross-repo reference ignored; plain issue comment unchanged; PR with no linkage → [PR]. (AC1–3, AC6, AC8)
• [x] T2 — _pr_entity + widened _CLOSING_KEYWORD_RE in cli/the_loop/webhook/router.py, with the cross-repo qualifier guard. (AC1, AC3)
• [x] T3 — linked_issue_numbers(entity, owner, repo) — closingIssuesReferences → head branch → body keywords, deduplicated, never the entity's own number. (AC2)
• [x] T4 — extract_work_items: linked issues before the PR number for every PR-shaped event; non-PR events unchanged. (AC4, AC5, AC6)
• [x] T5 — Webhook integration test (test_webhook_routing_integration.py, Gherkin + requirement link): a labelled PR's conversation comment resumes the issue's session; no new session is registered. (AC4, AC8)
• [x] T6 — Failing GhClient/provider unit tests (cli/tests/test_poller.py): the PR listing requests closingIssuesReferences; an Unknown JSON field error downgrades exactly once and is not retried on the next call; an unrelated GhError still propagates; refs() puts the linked issue first. (AC7)
• [x] T7 — GhClient.list_labeled_prs: request + parse the field, with the latched one-time downgrade and warning. (AC7)
• [x] T8 — Provider plumbing: GhItem.linked_issues, WorkItem.raw["linkedIssues"], _item_payload injecting closingIssuesReferences. (AC2, AC7)
• [x] T9 — Poll integration test (test_poller_integration.py, Gherkin + requirement link): a labelled PR whose linked issue has a live session forwards the comment into it and spawns nothing. (AC4, AC8)
• [x] T10 — Green gates: ruff, pyright, pytest, markdownlint from the repo root (hooks.prePush parity).
• [x] T11 — Docs: update docs/capabilities/webhook-triggers.md (current behaviour + history row) and record the routing-precedence decision in docs/decisions/.
• [x] T12 — PR + reviewer briefing from skills/the-loop/templates/pr-briefing.md, phase label → loop:needs-review.