workflow: meta rides the seam as data — the engine never evaluates it

P1 review finding: extractMeta timed only the literal's vm evaluation;
materializing the RESULT then read properties ordinarily on the HOST
stack, so a meta literal smuggling a getter (get name() { while(true){} })
could wedge the host outside any timeout — defeating the exact spin
isolation the worker thread exists for.

Rather than harden the evaluator (descriptor walks, AST validation),
delete the mechanism: the workflow's identity now reaches the seam as a
plain JSON field (WorkflowStartRequest.meta), carried by the tool as a
schema-validated `meta` object parameter the model fills directly. The
engine only shape-validates data (validateMeta, every violation named)
and pre-parses the body; the scanner, the vm evaluation, and the
host-side materialization are gone, and with them the hole. A body
still opening with a Claude Code-style `export const meta` statement
gets a pointed SCRIPT_PARSE message (the likeliest authoring slip; a
CC script's body stays drop-in, only its meta header moves into the
parameter). syncTimeoutMs now governs exactly one thing: the initial
synchronous slice inside the worker.

The RFC's decision section is rewritten in place (implemented-RFC
rule); the embedded-meta format moves to alternatives-considered with
the hole as the reason. Tool description, presentation (title now reads
meta.name directly — the textual sniff is gone), seam vocabulary docs,
and catalogs follow.

packages/workflow/workflow-workerthread/src/runtime.ts

@@ -109,7 +109,7 @@ export class WorkflowExecution {
     // wrapper, so under one Node version this throw is unreachable in
     // production — the session still maps it to an error result defensively.
     // lineOffset compensates for the wrapper line, so stack traces carry the
-    // script's own line numbers (the meta statement was blanked, not removed).
+    // script's own line numbers.
     try {
       this.compiled = new vm.Script(`(async () => {\n${body}\n})()`, {
         filename: `workflow:${meta.name}`,