fix: walk contentSchema, await the error envelope, correct the README claim

Copilot review round 7.

- Add `contentSchema` to the traversal. It holds the schema the decoded
  string content must satisfy, so a bare `true` or a remote `$ref` under
  it is a real finding all three clients were walking past. Also admit
  the three content keywords to `CONSTRAINING_KEYWORDS` on the same
  documented grounds as `format` — annotation-only by vocabulary, but
  plainly an attempt to constrain, so reporting such a schema as
  accepting anything would be noise. The comment now names that
  category rather than singling `format` out.

- Make `handleError` async and await the envelope write. Round 6 made
  the `--strict` report await its write and stopped one line short: the
  envelope emitted for the very same throw still raced `process.exit()`,
  so the machine-readable line a CI caller parses could be lost on a
  pipe. The launcher's comment claiming the envelope was "well inside
  the pipe buffer" was wrong reasoning and is replaced — fitting in the
  buffer says nothing about whether the write was performed. Its call
  site now awaits; the bin's `.catch(handleError)` needed no change.

- The CLI README called its sample block the "complete output" when the
  error envelope follows it on every non-zero exit. Now scoped to "the
  complete report", with the envelope named and linked.

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/core/schemaLint.test.ts

@@ -212,6 +212,20 @@ describe("lintToolSchemas — boolean-schema", () => {
       { type: "object", propertyNames: true, properties: {} },
       "inputSchema.propertyNames",
     ],
+    [
+      "contentSchema",
+      {
+        type: "object",
+        properties: {
+          a: {
+            type: "string",
+            contentMediaType: "application/json",
+            contentSchema: true,
+          },
+        },
+      },
+      "inputSchema.properties.a.contentSchema",
+    ],
     [
       "a dependentSchemas entry",
       { type: "object", dependentSchemas: { a: true }, properties: {} },
@@ -402,6 +416,9 @@ describe("lintToolSchemas — untyped-schema", () => {
     ["contains", { contains: { type: "string" } }],
     ["propertyNames", { propertyNames: { pattern: "^a" } }],
     ["dependentRequired", { dependentRequired: { a: ["b"] } }],
+    ["contentMediaType", { contentMediaType: "application/json" }],
+    ["contentEncoding", { contentEncoding: "base64" }],
+    ["contentSchema", { contentSchema: { type: "object" } }],
   ])("does not flag a schema constrained only by %s", (_label, sub) => {
     const findings = lintToolSchemas(
       tool({ inputSchema: { type: "object", properties: { a: sub } } }),