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/launcher/src/index.ts

@@ -79,10 +79,12 @@ async function run(): Promise<void> {
       // this `undefined`; fall through to the generic sink with the real message
       // rather than throwing "handleError is not a function" over it.
       if (typeof handleError !== "function") throw err;
-      // write-then-exit like the direct bin; the envelope is a few hundred bytes,
-      // well inside the pipe buffer, so the truncation risk noted above (which is
-      // about a large stdout payload) doesn't apply to this stderr line.
-      handleError(err); // writes the envelope + process.exit(code); never returns
+      // Write-then-exit like the direct bin — and AWAIT it. The envelope being
+      // a few hundred bytes is not the safeguard it looks like: on a pipe the
+      // write is asynchronous, so `process.exit()` discards it whether or not
+      // it would have fitted in the buffer. `handleError` settles on the write
+      // callback before exiting; skipping the await puts the truncation back.
+      await handleError(err); // writes the envelope + process.exit(code)
     }
   } else {
     const { runTui } = await import(clientEntry("tui"));