fix: verify scraped PR URLs against the git platform before pr_opened

[jonwiggins]

Summary

Fixes #531 — PR detection accepted any same-repo /pull/N URL in agent output as the task's opened PR, even when no PR was opened. As @genisd noted, even a PR URL pasted into the prompt (and echoed back by the agent) would immediately flip the task to pr_opened, pointing at a PR that may not exist or belongs to someone else.

Root cause

task-worker scans streamed agent output for PR/MR URLs and, at exec end, trusted any candidate that merely matched the task's repo (capturedPrUrl / persisted tasks.prUrl / raw-log fallback) — no check that the PR exists, is open, or was created from the task's branch.

Fix — verify against the platform before trusting the URL

Task branches are deterministic (optio/task-<taskId>), so the platform's answer for that branch is a strong, authoritative signal. Before the pr_opened transition, the worker now calls the existing GitPlatform.listOpenPullRequests(ri, { branch }) (already implemented with head/source-branch filtering for GitHub, GitLab, and CodeCommit) via a new verifyTaskPr():

• Verified (open PR with head = task branch): transition to pr_opened using the canonical URL from the platform (wins over the scraped URL, which may be truncated or wrong).
• No PR on the branch: the scraped URL is ignored with a logged skip, and any prUrl persisted mid-stream is cleared (clearTaskPr). The task then flows to needs_attention with reason completed_without_pr — the behavior the issue expected. The verification result is reused so the existing API-fallback path doesn't repeat the same platform call.
• Platform unavailable (no git token, unparseable repo URL, or API error): falls back to the previous trust-the-logs behavior rather than breaking detection for that configuration. This is the graceful-degradation path — verification never turns a working setup into a broken one.

checkExistingPr (pre-agent short-circuit, API fallbacks, reconciliation) is now a thin wrapper over verifyTaskPr, so all platform lookups share one implementation.

Changes

• apps/api/src/services/pr-detection-service.ts — new verifyTaskPr() (discriminated verified / no_pr / unavailable result) and pure resolveDetectedPrUrl() decision helper; checkExistingPr() refactored on top
• apps/api/src/workers/task-worker.ts — verify scraped URL before the pr_opened transition; skip redundant API-fallback calls when the platform already reported no PR
• apps/api/src/services/task-service.ts — clearTaskPr() to remove a bogus prUrl/prNumber persisted during streaming
• apps/api/src/services/pr-detection-service.test.ts — tests: prompt-echoed URL rejected when no PR exists on the branch; genuine PR with matching head branch accepted; canonical-URL preference; unavailable-platform fallback

Testing

• pnpm turbo typecheck — 12/12 packages pass
• apps/api vitest suite — 122 files, 2156 tests pass (includes new verification tests)
• pnpm format:check — clean