fix: map the new exit code, keep add runnable, distinguish severity by shape

Copilot review round 4.

- Add `SCHEMA_UNPORTABLE` to `codeForExit`. `CliExitCodeError` lets a
  caller omit the envelope, so without the case an envelope-less throw
  reported the generic `error` — the same silent mislabelling the rename
  was meant to prevent, one layer down. Covered via the envelope-less
  construction that exposed it.

- Keep a local `type` (and the preset's `required`) alongside the remote
  `$ref` in the showcase config. The TUI's `schemaToForm` dispatches on
  `type`, so a `$ref`-only property fell through to a string field that
  `add`'s numeric handler then rejected — making the README's "every
  showcase tool is runnable" claim false there. The `remote-ref` finding
  is unaffected, since that rule reads `$ref` independently.

- Restrict `type-union` to well-formed unions: non-empty, unique, and
  all recognized JSON Schema type names. Its message asserts the
  construct is *legal* JSON Schema, which is false for `[]`, `[3]` or
  `["bananas"]`, and the `anyOf` it suggested for those would be invalid
  too. Malformed schemas stay the SDK parser's business.

- Drop `if`/`then`/`else` from the presence-only allowlist. They only
  constrain as a pair, so `{"if": {"const": 1}}` accepts every value and
  was evading `untyped-schema`; the pairing is now checked explicitly.

- Distinguish the web sidebar severities by SHAPE, not just colour — a
  circle for an error, a triangle for a warning — and name the severity
  in the tooltip. Colour alone left a colour-blind reader unable to tell
  them apart.

- Fix the stale "across 2 tools" count in the CLI README's hint sample.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UL9retfvAXRgvi6EWk4SY4
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/integration/mcp/raw-tool-schemas.test.ts

@@ -256,6 +256,26 @@ describe("rawToolSchemas override (#1005)", () => {
       { tool: "add", rules: ["remote-ref"] },
     ]);
     expect(countFindings(results)).toEqual({ errors: 1, warnings: 3 });
+
+    // Every flagged tool in the fixture must stay runnable — the README and
+    // the PR say so, and a showcase that errors on its own obvious button is
+    // worse than no showcase. Each is called with the arguments its ADVERTISED
+    // schema renders, including the properties the override added, since that
+    // is what a user clicking through the form would send.
+    const byName = (n: string) => tools.find((t) => t.name === n)!;
+    await expect(
+      connected.callTool(byName("get_temp"), { city: "Oslo", units: "C" }),
+    ).resolves.toBeDefined();
+    await expect(
+      connected.callTool(byName("echo"), {
+        message: "hi",
+        show_ids: null,
+        opts: {},
+      }),
+    ).resolves.toBeDefined();
+    await expect(
+      connected.callTool(byName("add"), { a: 1, b: 2 }),
+    ).resolves.toBeDefined();
   });
 
   it("keeps a structured-content tool callable under an outputSchema override", async () => {