feat(setup-gbrain): carve the branch-exclusive install paths into sections

Brain-init (Paths 1/2/3/4 bodies), engine remediation, transcript gate, and
CLAUDE.md persist load on demand — at most one install route ever runs.
Skeleton 75.3KB -> 57.0KB; the Step 1 detect and Step 2 path dispatch stay
always-loaded. New buildSetupGbrainFixture helper gives the periodic E2Es
extract-don't-copy fixtures with a non-empty guard; the voyage-code-3 gate
counts scan the tmpl union (the third init site lives in engine-remediation).

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

setup-gbrain/sections/transcript-gate.md

@@ -0,0 +1,66 @@
+<!-- AUTO-GENERATED from transcript-gate.md.tmpl — do not edit directly -->
+<!-- Regenerate: bun run gen:skill-docs -->
+After memory sync is wired (Step 7) but before persisting the CLAUDE.md
+config (Step 8), offer to bring this Mac's coding-agent transcripts +
+curated `~/.gstack/` artifacts into gbrain so the retrieval surface
+(per-skill manifests, salience block) has data to surface.
+
+Run the probe to size the operation:
+```bash
+bun run ~/.claude/skills/gstack/bin/gstack-memory-ingest.ts --probe
+```
+
+Read the output. If `Total files in window: 0`, skip — there's nothing
+to ingest. Set `gstack-config set transcript_ingest_mode incremental`
+silently and continue to Step 8.
+
+If `New (never ingested)` is < 200 AND total bytes are < 100MB: silent
+bulk via `bun run ~/.claude/skills/gstack/bin/gstack-memory-ingest.ts --bulk --quiet`. Set
+`transcript_ingest_mode=incremental` and continue.
+
+Otherwise (the "many transcripts on disk" path): AskUserQuestion with
+the exact counts AND the value promise. Default scope is **current repo
+only, last 90 days**:
+
+> "Found <N_repo> transcripts in THIS repo (<repo-slug>) over the last
+> 90 days, plus <N_other> across other repos on this machine (<bytes>
+> total if all ingested). Ingest THIS repo's transcripts into gbrain?
+>
+> What you get after this: every gstack skill auto-loads recent salience
+> from your past sessions in this repo, so the agent finds your prior
+> work without you describing it. You can query 'what was I doing on
+> day X' and get a real answer. Per-session pages are searchable,
+> taggable, and deletable. Secret scanning runs before any push.
+>
+> What stays the same: nothing leaves your machine unless gbrain sync
+> is enabled (Step 7). Per-repo trust policies still apply.
+>
+> Multi-Mac note: if you HAVE enabled brain sync (Step 7), these
+> transcript pages will sync across your Macs. Caveat: deleting a
+> transcript page later removes it from gbrain but git history retains
+> it in prior commits. Use `gstack-transcript-prune` to delete in bulk;
+> use `git filter-repo` on the brain remote for hard-delete from
+> history."
+
+Options:
+- A) Yes — this repo, last 90 days (recommended; ~est min)
+- B) Yes — this repo, ALL history
+- C) Yes — this repo + other repos on this machine
+- D) Skip historical, track new from now (`transcript_ingest_mode=incremental`)
+- E) Never ingest transcripts (`transcript_ingest_mode=off`)
+
+After answer:
+```bash
+~/.claude/skills/gstack/bin/gstack-config set transcript_ingest_mode <choice>
+bun run ~/.claude/skills/gstack/bin/gstack-gbrain-sync.ts --full --no-brain-sync
+```
+(`--no-brain-sync` because Step 7 already wired that path; this just
+runs the code import + memory ingest stages. Brain-sync will run on the
+next preamble hook.)
+
+If A/D/E, ingest is incremental from this point on; preamble-boundary
+hook runs `bun run ~/.claude/skills/gstack/bin/gstack-gbrain-sync.ts --incremental --quiet` on every skill
+start (cheap mtime fast-path).
+
+Reference doc for users: `setup-gbrain/memory.md` (linked from CLAUDE.md
+Step 8).