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/components/groups/ToolListItem/ToolListItem.tsx

@@ -8,7 +8,7 @@ import {
   UnstyledButton,
 } from "@mantine/core";
 import { useMemo } from "react";
-import { RiErrorWarningLine } from "react-icons/ri";
+import { RiAlertLine, RiErrorWarningLine } from "react-icons/ri";
 import type { Tool } from "@modelcontextprotocol/client";
 import { lintToolSchemas } from "@inspector/core/json/schemaLint.js";
 import { resolveDisplayLabel } from "../../../utils/toolUtils";
@@ -87,7 +87,7 @@ export function ToolListItem({ tool, selected, onClick }: ToolListItemProps) {
         </ItemBody>
         {findings.length > 0 && (
           <Tooltip
-            label={`${findings.length} schema portability finding${findings.length === 1 ? "" : "s"} — select the tool for detail`}
+            label={`${findings.length} schema portability ${hasError ? "error" : "warning"}${findings.length === 1 ? "" : "s"} — select the tool for detail`}
             withArrow
           >
             <SchemaFlagIcon
@@ -102,7 +102,11 @@ export function ToolListItem({ tool, selected, onClick }: ToolListItemProps) {
                   : "Schema portability warnings"
               }
             >
-              <RiErrorWarningLine />
+              {/* Distinct SHAPES, not just distinct colours: an octagon for an
+                  error and a triangle for a warning, so the severity survives
+                  for a colour-blind reader. The tooltip and the accessible
+                  label name it in words as well. */}
+              {hasError ? <RiErrorWarningLine /> : <RiAlertLine />}
             </SchemaFlagIcon>
           </Tooltip>
         )}