feat: show issue labels in issue view and add its --fields flag

`issue view` rendered state but never labels, and offered no way to add
them — so reading one issue's labels forced a detour through
`issue list --fields labels` and hunting the matching row. The benchmark
transcripts showed agents paying this round-trip on every labels/state
read.

Show labels by default in the detail view (a detail view should be
complete), and add a `--fields` flag mirroring `issue list` / `search`
to append assignees, closedAt, milestone, updatedAt, url on request.

Also strengthen SKILL.md against the two command-discovery round-trips
the transcripts exposed: name the required `search issues` / `search prs`
subcommand form (a bare `search "<query>"` is invalid), and point agents
straight at `issue view <n>` for a single issue's fields.

Verified live: read-issue-labels-and-state dropped from 10 turns to 4
(cache-read ~3.3x lower), the transcript reduced to three clean commands
with the search-help and issue-list round-trips gone.

test/issue-view.test.ts

@@ -47,6 +47,46 @@ describe("issue view", () => {
     expect(stdout).toContain("comment_count: 3 — use --comments to see full comments");
   });
 
+  it("renders the issue's labels comma-joined by default, with no flag", async () => {
+    server = await startFixtureServer([
+      {
+        method: "GET",
+        path: ISSUE_PATH,
+        body: issueBody({ labels: [{ name: "bug" }, { name: "regression" }] }),
+      },
+    ]);
+    const { stdout, exitCode } = await runCliTest(["issue", "view", "42"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    // TOON-quoted because the joined value contains a comma.
+    expect(stdout).toContain('labels: "bug, regression"');
+  });
+
+  it("appends named extra fields with --fields on top of the default fields", async () => {
+    server = await startFixtureServer([
+      {
+        method: "GET",
+        path: ISSUE_PATH,
+        body: issueBody({
+          assignees: [{ login: "alexion" }],
+          milestone: { title: "v2.0" },
+        }),
+      },
+    ]);
+    const { stdout, exitCode } = await runCliTest(
+      ["issue", "view", "42", "--fields", "assignees,milestone"],
+      { env: testModeEnv(server.url) },
+    );
+
+    expect(exitCode).toBe(0);
+    // Default fields are still present; the extra fields are appended.
+    expect(stdout).toContain("state: open");
+    expect(stdout).toContain("assignees: alexion");
+    expect(stdout).toContain("milestone: v2.0");
+  });
+
   it("renders comment_count: 0 when there are no comments", async () => {
     server = await startFixtureServer([
       { method: "GET", path: ISSUE_PATH, body: issueBody({ comments: 0 }) },