fix(linear): don't treat a linked github issue as a PR

`findLinkedGitHubPr` accepted any attachment with `sourceType: "github"` as a
PR. Linear's "GitHub Issues" sync tags the source issue with that same
sourceType (URL `.../issues/N`, no branch metadata), so the oneshot seed
classified it as a linked PR, set `source: "github-integration"`, and fell
back to `issue.branchName` in `mode: "existing"`. That branch was never
created, so `createWorktree` threw `Branch not found` and every GIT-team
oneshot was dropped.

Only trust the explicit PR source types; for the generic `github` type require
a `/pull/` URL. GitHub-issue attachments now fall through to `source: "none"`
→ `mode: "new"`, so webmux creates the branch fresh like any other issue.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rubenfiszel

backend/src/__tests__/linear-service.test.ts

@@ -461,6 +461,21 @@ describe("findLinkedGitHubPr", () => {
     expect(result?.url).toBe("https://github.com/org/repo/pull/42");
     expect(result?.state).toBe("unknown");
   });
+
+  it("ignores a linked github issue tagged sourceType 'github' (not a PR)", () => {
+    // Linear's "GitHub Issues" sync attaches the source issue with
+    // sourceType "github" and an /issues/ URL. It must not be treated as a PR,
+    // else the oneshot seed resolves to a never-created branch.
+    const result = findLinkedGitHubPr({
+      attachments: [
+        attachment({
+          id: "1", url: "https://github.com/org/repo/issues/10105", title: "#10105 bug",
+          createdAt: "2026-05-01", sourceType: "github",
+        }),
+      ],
+    });
+    expect(result).toBeNull();
+  });
 });
 
 describe("buildLinearPickupMarkdown", () => {

backend/src/services/linear-service.ts

@@ -842,9 +842,13 @@ export function findLinkedGitHubPr(
   issue: { attachments: LinearAttachment[] },
 ): LinkedGitHubPr | null {
   const githubAttachments = issue.attachments.filter((a) => {
-    if (a.sourceType === "github" || a.sourceType === "githubPR" || a.sourceType === "github_pull_request") {
+    if (a.sourceType === "githubPR" || a.sourceType === "github_pull_request") {
       return true;
     }
+    // `sourceType: "github"` covers both linked PRs and linked issues, so it is
+    // not enough on its own — a synced GitHub *issue* (Linear's "GitHub Issues"
+    // team) would otherwise be treated as a PR, and the oneshot seed resolves to
+    // a branch that was never created ("Branch not found"). Only /pull/ URLs are PRs.
     return /github\.com\/.+\/pull\/\d+/i.test(a.url);
   });
   if (githubAttachments.length === 0) return null;