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>

sync-gbrain/SKILL.md.tmpl

@@ -101,7 +101,9 @@ BEFORE invoking the orchestrator:
   "Your brain queries (the `mcp__gbrain__*` tools) work via remote MCP, but
   symbol code search needs a local PGLite. Run `/setup-gbrain` and pick
   'Yes' at the new 'local code index' prompt (Step 4.5), or run
-  `gbrain init --pglite --json` directly. Continuing without code stage."
+  `gbrain init --pglite --json --embedding-model voyage:voyage-code-3 --embedding-dimensions 1024`
+  directly (drop the voyage flags if `VOYAGE_API_KEY` isn't set). Continuing
+  without code stage."
   Then proceed to Step 2 — the orchestrator's `runCodeImport()` and
   `runMemoryIngest()` will return SKIP per plan D12; only `runBrainSyncPush()`
   will run. Do NOT abort.
@@ -114,7 +116,8 @@ BEFORE invoking the orchestrator:
     1. Re-run /setup-gbrain — Step 1.5 offers Retry / Switch to PGLite /
        Switch brain mode / Quit (plan D4).
     2. Repair manually: mv ~/.gbrain/config.json ~/.gbrain/config.json.bak
-       && gbrain init --pglite --json
+       && gbrain init --pglite --json --embedding-model voyage:voyage-code-3 \
+          --embedding-dimensions 1024   (drop voyage flags if VOYAGE_API_KEY unset)
   Re-run /sync-gbrain after.
   ```
   Do NOT continue — the orchestrator would skip code+memory and only run