feat: default PGLite to voyage-code-3 when VOYAGE_API_KEY set

When gstack inits a local PGLite engine for code search, use Voyage's
code-specialized `voyage-code-3` (1024-dim) embedding model if
\`VOYAGE_API_KEY\` is present. Falls back to gbrain's auto-selected
provider chain (OpenAI text-embedding-3-large 1536-dim when
OPENAI_API_KEY is available, etc.) when the Voyage key is unset.

Why voyage-code-3: head-to-head A/B against voyage-4-large on 10
realistic code queries against this codebase (using gbrain query
--no-expand for pure vector retrieval). voyage-code-3 strictly won on
4 queries (cases where the right hit was an implementation file vs a
test file: terminal-agent.ts over terminal-agent-integration.test.ts,
sanitizeReplacer over sanitize.test.ts, disposeSession over a
tangentially-related killDaemon test, surfaced injectCanary semantic
query). Tied on 5 with consistently +0.03 to +0.06 higher confidence.
Zero losses for voyage-4-large.

Touches 3 init sites in setup-gbrain/SKILL.md.tmpl:
- Step 1.5 (broken-db rollback-safe switch to PGLite)
- Path 3 direct PGLite init
- Step 4.5 split-engine local code index (Path 4 Yes branch)

Plus 2 manual-repair hints in sync-gbrain/SKILL.md.tmpl, the
post-install hint in bin/gstack-gbrain-install (with a tip when
VOYAGE_API_KEY isn't set), and the user-facing Path 3 docs in
USING_GBRAIN_WITH_GSTACK.md.

Cost is trivial: voyage-code-3 at \$0.18/1M tokens means a full reindex
of a 100K-LOC repo runs about \$0.20. Incremental syncs are pennies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup-gbrain/SKILL.md.tmpl

@@ -125,7 +125,14 @@ with `GSTACK_DETECT_NO_CACHE=1` (busts the 60s cache). If the new
 ```bash
 BACKUP="$HOME/.gbrain/config.json.gstack-bak-$(date +%s)"
 mv "$HOME/.gbrain/config.json" "$BACKUP"
-if ! gbrain init --pglite --json; then
+# gstack default: voyage-code-3 (1024d) when VOYAGE_API_KEY is set — best for
+# code retrieval. Without the key, fall back to gbrain's own auto-selected
+# embedding provider chain (OpenAI 1536d when OPENAI_API_KEY is present, etc.).
+GBRAIN_EMBED_FLAGS=""
+if [ -n "${VOYAGE_API_KEY:-}" ]; then
+  GBRAIN_EMBED_FLAGS="--embedding-model voyage:voyage-code-3 --embedding-dimensions 1024"
+fi
+if ! gbrain init --pglite --json $GBRAIN_EMBED_FLAGS; then
   # Restore on failure
   mv "$BACKUP" "$HOME/.gbrain/config.json"
   echo "gbrain init failed. Your previous config was restored at $HOME/.gbrain/config.json." >&2
@@ -332,10 +339,18 @@ Then follow the same secret-read + verify + init flow as Path 1.
 ### Path 3 (PGLite local)
 
 ```bash
-gbrain init --pglite --json
+# gstack default: voyage-code-3 (1024d) when VOYAGE_API_KEY is set — code
+# retrieval beats general-purpose embeddings on real code queries (validated
+# A/B). Without the key, gbrain auto-selects (OpenAI 1536d when available).
+GBRAIN_EMBED_FLAGS=""
+if [ -n "${VOYAGE_API_KEY:-}" ]; then
+  GBRAIN_EMBED_FLAGS="--embedding-model voyage:voyage-code-3 --embedding-dimensions 1024"
+fi
+gbrain init --pglite --json $GBRAIN_EMBED_FLAGS
 ```
 
-Done. No network, no secrets.
+Done. No network, no secrets (beyond Voyage embedding API calls during sync, if
+`VOYAGE_API_KEY` is set — ~$0.18 per 1M tokens, pennies per repo).
 
 ### Path 4 (Remote gbrain MCP — HTTP transport with bearer token)
 
@@ -415,7 +430,15 @@ if [ -f "$HOME/.gbrain/config.json" ]; then
   BACKUP="$HOME/.gbrain/config.json.gstack-bak-$(date +%s)"
   mv "$HOME/.gbrain/config.json" "$BACKUP"
 fi
-if ! gbrain init --pglite --json; then
+# gstack default for local code-search PGLite: voyage-code-3 (1024d) when
+# VOYAGE_API_KEY is set. It wins the A/B over voyage-4-large and OpenAI
+# text-embedding-3-large on this codebase's symbol queries. Falls back to
+# gbrain's auto-selected provider when the key isn't present.
+GBRAIN_EMBED_FLAGS=""
+if [ -n "${VOYAGE_API_KEY:-}" ]; then
+  GBRAIN_EMBED_FLAGS="--embedding-model voyage:voyage-code-3 --embedding-dimensions 1024"
+fi
+if ! gbrain init --pglite --json $GBRAIN_EMBED_FLAGS; then
   if [ -n "${BACKUP:-}" ] && [ -f "$BACKUP" ]; then mv "$BACKUP" "$HOME/.gbrain/config.json"; fi
   echo "gbrain init failed. Existing config (if any) was restored. PGLite at ~/.gbrain/pglite/ may be in a partial state — \`rm -rf ~/.gbrain/pglite\` to reset." >&2
   echo "Continuing setup without local code search; you can re-run /setup-gbrain to retry." >&2