fix(ci): Add separate pgrx init steps for Ubuntu and macOS

macOS uses Homebrew path for PostgreSQL, not the Linux system path.
Split pgrx init into OS-specific steps with correct pg_config paths.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/postgres-extension-ci.yml

@@ -94,10 +94,16 @@ jobs:
       - name: Install cargo-pgrx
         run: cargo install cargo-pgrx --version 0.12.0 --locked
 
-      - name: Initialize pgrx
+      - name: Initialize pgrx (Ubuntu)
+        if: runner.os == 'Linux'
         run: cargo pgrx init --pg${{ matrix.pg_version }}=/usr/lib/postgresql/${{ matrix.pg_version }}/bin/pg_config
         working-directory: crates/ruvector-postgres
 
+      - name: Initialize pgrx (macOS)
+        if: runner.os == 'macOS'
+        run: cargo pgrx init --pg${{ matrix.pg_version }}=/opt/homebrew/opt/postgresql@${{ matrix.pg_version }}/bin/pg_config
+        working-directory: crates/ruvector-postgres
+
       - name: Check code formatting
         run: cargo fmt --all -- --check
         working-directory: crates/ruvector-postgres