feat: version-aware find-browse with META signal protocol

Agents in other workspaces found stale browse binaries that were missing
newer flags. find-browse now compares the local binary's git SHA against
origin/main via git ls-remote (4hr cache), and emits META:UPDATE_AVAILABLE
when behind. SKILL.md setup checks parse META signals and prompt the user
to update.

- New compiled binary: browse/dist/find-browse (TypeScript, testable)
- Bash shim at browse/bin/find-browse delegates to compiled binary
- .version file written at build time with git commit SHA
- Build script compiles both browse and find-browse binaries
- Graceful degradation: offline, missing .version, corrupt cache all skip check

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

setup

@@ -32,6 +32,10 @@ if [ "$NEEDS_BUILD" -eq 1 ]; then
     bun install
     bun run build
   )
+  # Safety net: write .version if build script didn't (e.g., git not available during build)
+  if [ ! -f "$GSTACK_DIR/browse/dist/.version" ]; then
+    git -C "$GSTACK_DIR" rev-parse HEAD > "$GSTACK_DIR/browse/dist/.version" 2>/dev/null || true
+  fi
 fi
 
 if [ ! -x "$BROWSE_BIN" ]; then