Port /impeccable generate to the engine crates

The Node-era server, CLI, hook, and pin halves of the generate command move
into the Rust workspace, with the protocol unchanged:

- crates/live: POST /agent-target is held open on a channel plus a timer
  thread (the manual-apply deferred pattern), releasing its turnstile
  ticket before it parks like /poll; /agent-target-result resolves it;
  /agent-target-claim is the roll call with its renewable lease. SSE
  connections carry the overlay's clientId: a late overlay is replayed
  every pending target, and a disconnect retires that overlay's report,
  releases its lease, and re-judges each roll call. Shutdown drains held
  requests with server_stopping.
- crates/live/src/live_generate.rs: the live-generate verb (the router
  already forwards every live* verb), same flags, verdicts, and
  _instructions, spelled with the engine's self command.
- crates/hook: every entry stands down on live preview markers
  (skipped: live-preview), checking the proposed content and the file on
  disk for hook-before-edit.
- crates/context: pin accepts generate; the crate's command-metadata.json
  copy carries its entry.

Tests: crates/cli/tests/agent_target.rs (six HTTP cases with an SSE reader),
tests/live-agent-target.test.mjs rewritten to drive the binary (28 cases,
registered in the live suite), hook stand-down cases, oracle goldens for
live-generate plus the re-recorded pin list goldens, the e2e prompt
assertion waiting for the journaled event, and the contract documented in
docs/CLI-CONTRACT.md.

AI-assisted: implemented and tested with Claude Code under maintainer
direction.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/skill-behavior/scenarios.test.mjs

@@ -31,6 +31,7 @@ import {
 import { detectProvider, getModel, hasKey, resolveModelList, PROVIDERS } from './providers.mjs';
 import { assertLauncherDenialWarningBeforeNextTool, assertPlanningFallbackWarning, LAUNCHER_FAILURE_WARNING, assertAdviceOnly, assertWorkflowAdvice, assertCommandComparison, missingReferences } from './assertions.mjs';
 import { assertCompleted } from '../skill-workflow/assertions.mjs';
+import { findEngineBinary } from '../lib/engine-bin.mjs';
 import {
   PRODUCT_MD_SAMPLE,
   PRODUCT_MD_SAMPLE_NO_REGISTER,
@@ -106,10 +107,16 @@ function loadedBefore(trace, first, second) {
  */
 function stopLiveHelper(workspace) {
   try {
+    const engineBin = findEngineBinary();
     execFileSync(
-      process.execPath,
-      [path.join(workspace, '.claude/skills/impeccable/scripts/live-server.mjs'), 'stop'],
-      { cwd: workspace, stdio: 'ignore', timeout: 10_000 },
+      path.join(workspace, '.claude/skills/impeccable/scripts/impeccable'),
+      ['live-server', 'stop'],
+      {
+        cwd: workspace,
+        stdio: 'ignore',
+        timeout: 10_000,
+        env: { ...process.env, ...(engineBin ? { IMPECCABLE_BIN: engineBin } : {}) },
+      },
     );
   } catch { /* nothing was running */ }
 }