feat: report a skill-name collision across the listing (#2248)

SEP-2640: hosts MUST NOT assume name uniqueness, and when two entries in one
listing collide on `name` a host MUST disambiguate them rather than silently
discarding or preferring one.

`checkSkillConformance` structurally cannot see this — it takes one entry and
a collision is a property of the pair — so `checkSkillNameCollisions` walks
the listing and returns a finding per colliding entry, each naming the
others. All three clients merge it into the entry's own findings, so it
carries through the header badge, the CLI report and the TUI row marks with
no new surface.

**It is a `warning`, not an `error`,** and that is the severity split doing
its job. The obligation is on the *consumer*: a server may legitimately
publish two skills with the same name under different paths, and the SEP's
own `acme/billing/refunds` example is exactly that shape. Calling it an error
would tell a conforming author their catalog is invalid. `--verify` therefore
still exits 0 for a collision.

In the web screen it renders as a banner directly under the Conformance
header and is filtered out of the findings list, the same treatment
`dynamic-resources` gets: it changes how everything below it should be read,
and stating one fact twice reads as two findings.

Fixed while adding it: an entry whose only finding was a banner one rendered
an EMPTY findings container instead of "no structural issues", because the
list's presence was decided on the unfiltered set while its contents were
filtered. `listedIssues` is now derived once and used for both.

The fixture gains `acme/reports` + `globex/reports`, both fully conforming
and sharing the name `reports` — also the only fixture with a multi-segment
skill path, which nothing else exercised. Eight skills over four pages now,
and the integration test walks the cursor to exhaustion rather than asserting
a fixed page count, so a future fixture does not require editing it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ND46Ci64DC3tzBy9aG81Nc
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/tui/__tests__/SkillsTab.test.tsx

@@ -602,6 +602,51 @@ describe("SkillsTab (#2248)", () => {
     expect(lastFrame() ?? "").toContain("nameless");
   });
 
+  it("reports a name collision on both entries, as a warning", async () => {
+    // A catalog-level fact `checkSkillConformance` structurally cannot see —
+    // and a warning, because the server did nothing wrong: the obligation is
+    // on the consumer to tell two same-named skills apart.
+    const acme: SkillEntry = {
+      uri: "skill://acme/reports/SKILL.md",
+      frontmatter: { name: "reports", description: "Acme ledger" },
+      resources: [
+        { uri: "skill://acme/reports/SKILL.md", digest: CLEAN_DIGEST, size: 1 },
+      ],
+    };
+    const globex: SkillEntry = {
+      uri: "skill://globex/reports/SKILL.md",
+      frontmatter: { name: "reports", description: "Globex ledger" },
+      resources: [
+        {
+          uri: "skill://globex/reports/SKILL.md",
+          digest: CLEAN_DIGEST,
+          size: 1,
+        },
+      ],
+    };
+    const { lastFrame, stdin } = render(
+      <SkillsTab
+        skills={[acme, globex]}
+        pageCount={1}
+        inspectorClient={null}
+        width={160}
+        height={30}
+        focusedPane="list"
+      />,
+    );
+    const frame = lastFrame() ?? "";
+    // Both rows carry the warning mark, not the error one.
+    expect(frame.match(/! reports/g)).toHaveLength(2);
+    expect(frame).not.toContain("✗ reports");
+    expect(frame).toContain("also declares the name");
+    // The detail pane names the OTHER skill, which is the disambiguation.
+    expect(frame).toContain("skill://globex/reports/SKILL.md");
+
+    stdin.write(DOWN);
+    await tick();
+    expect(lastFrame() ?? "").toContain("skill://acme/reports/SKILL.md");
+  });
+
   it("shows the details footer only when the details pane is focused", () => {
     const unfocused = render(
       <SkillsTab