fix(test): unique tmp dirs for plan artifacts + audited live-repo cwd sites

Six paid PTY tests wrote their expected plan artifact to a FIXED shared
/tmp path ('/tmp/gstack-test-plan-<mode>.md') and rmSync'd it in
finally — under --retry 1, EVALS_JOBS>1, or two concurrent worktrees, a
sibling's cleanup deletes this run's artifact and the D19 'agent did
not produce expected plan file' assertion fires spuriously. Each test
now mkdtemps its own dir, interpolates the unique path into the agent
prompt (fixture-sourced prompts get a replaceAll + drift guard that
throws if the fixture's literal ever moves), and cleans up its own dir.

The 18 cwd:-into-the-live-repo sites were audited: all deliberate
(skill registry + hermetic pre-trusted dir, in-repo gen renders, git
history reads, slug resolution) — each now carries a
'// LIVE-REPO CWD: <reason>' comment so the next audit can tell
deliberate from accidental.

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

test/skill-e2e-preamble-script-ab.test.ts

@@ -40,6 +40,8 @@ const INLINE_REF = '29785978'; // last pre-Phase-1 commit (v1.69.1.0 bump)
 
 function inlineSkill(): string {
   return execSync(`git show ${INLINE_REF}:plan-ceo-review/SKILL.md`, {
+    // LIVE-REPO CWD: git show needs this repo's history to read the
+    // pre-Phase-1 SKILL.md render at INLINE_REF.
     cwd: ROOT,
     encoding: 'utf-8',
     maxBuffer: 8 * 1024 * 1024,