fix: pre-landing review fixes for the Aside-first branch

Review army + adversarial passes (Claude and Codex) on the merged branch:

setup
- _prune_stale_generated scans the host dirs too (the generator already
  removed the render before setup ran, so the host branch was dead), skips
  symlinks in the render tree (rm -rf on a slash-terminated link empties its
  target), removes a host symlink only when it resolves into gstack, cleans a
  bannered real dir through _cleanup_weak_dir, recognizes frontmatter-renamed
  skills, and logs through log. The always-run codex render passes every host
  dir that may link to it.
- NEEDS_BUILD checks all three binaries (with $_EXE) and lib/ sources; the
  browser hint and the bootstrap summary honor GSTACK_SKIP_ASIDE, treat a
  requested skip as a request, and derive one skill list.

lib/aside-render.ts + bin/gstack-render.ts
- The loopback server carries a per-render secret path, checks containment on
  the real path (symlink escapes are 403), and rejects malformed encoding.
- Inline eval results are one base64 line, so page text cannot forge
  ASIDE_DIR= or the sentinel; the last ASIDE_DIR wins.
- runProc escalates SIGTERM to SIGKILL, bounds every wait, and clears every
  timer (an uncleared one kept gstack-render alive after printing OK).
- renderTmpDir refuses a shared /tmp name owned by someone else; the work dir
  and server are created inside try; goto's budget follows the render budget.
- probeAside classifies a present-but-failing CLI as ASIDE_NOT_RUNNING like
  the skills' bash probe; render() retries on gstack's own browser when Aside
  could not start or its private CDP bridge is gone (never on a page error
  or a timeout of a running script); the CLI reports the engine that actually
  rendered, exits 0 on --help, rejects non-numeric flags, documents
  --wait-timeout, fences EVAL/PAGE_ERRORS as untrusted content, and names the
  daemon's cookie-import JS lock remedy.
- The browse path passes --scale only when asked (a scale change rebuilds
  the daemon context) and restores the viewport after a sized screenshot.

resolvers / templates
- The bash probe honors GSTACK_SKIP_ASIDE and has a perl deadline on stock
  macOS; .local is no longer LOCAL (mDNS); same-origin filters compare parsed
  origins; link status is HEAD-checked only on LOCAL targets; every
  aside exec goes through the receipted _aside_exec prelude
  ({{ASIDE_EXEC_PRELUDE}}), including nine template blocks that called it
  bare; the design sketch and diagram staging use private directories.
- The generator prunes only bannered renders and never a host whose
  generation failed.

Docs, stale comments and dead code cleaned; goldens re-rendered; tests
updated and added for every behavior above.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

scripts/resolvers/design.ts

@@ -530,11 +530,13 @@ Generate a single-page HTML file with these constraints:
   matches the actual use case)
 - Add HTML comments explaining design decisions
 
-Write it to \`/tmp/gstack-sketch/sketch.html\` (Write tool) — its own directory,
-because the renderer serves that directory over loopback:
+Create a private directory for it first — the renderer serves that whole directory
+over loopback, so it must be yours alone and hold nothing else (never a fixed,
+shared /tmp name another user could pre-create):
 \`\`\`bash
-mkdir -p /tmp/gstack-sketch
+mktemp -d "\${TMPDIR:-/tmp}/gstack-sketch.XXXXXX"
 \`\`\`
+Write the sketch to \`<that directory>/sketch.html\` (Write tool).
 
 **Step 3: Render and capture**
 
@@ -543,7 +545,7 @@ in gstack's own headless browser (its first line says which: \`ENGINE=aside\` or
 \`ENGINE=browse\`) — and screenshots it:
 
 \`\`\`bash
-bun run ${toShellPath(ctx.paths.binDir)}/gstack-render.ts /tmp/gstack-sketch/sketch.html --screenshot /tmp/gstack-sketch.png --width 1280
+bun run ${toShellPath(ctx.paths.binDir)}/gstack-render.ts <sketch-dir>/sketch.html --screenshot <sketch-dir>/sketch.png --width 1280
 \`\`\`
 
 Only if it prints \`NEEDS_ASIDE\` or \`ASIDE_NOT_RUNNING\` followed by \`ERROR: no browser
@@ -562,7 +564,7 @@ If they approve or say "good enough," proceed.
 **Step 5: Include in design doc**
 
 Reference the wireframe screenshot in the design doc's "Recommended Approach" section.
-The screenshot file at \`/tmp/gstack-sketch.png\` can be referenced by downstream skills
+The screenshot file at \`<sketch-dir>/sketch.png\` (name the full path in the doc) can be referenced by downstream skills
 (\`/plan-design-review\`, \`/design-review\`) to see what was originally envisioned.
 
 **Step 6: Outside design voices** (optional)