fix(ci): add the cross-compile target to the pinned toolchain

The rust-toolchain.toml pin added in the previous commit broke the Intel Mac
release build: dtolnay/rust-toolchain@stable installs the target for stable,
but cargo then runs the pinned 1.97.1, which has no x86_64-apple-darwin std,
so the build failed with 'can't find crate for core'.

Add an explicit 'rustup target add' step after the toolchain install in both
ci.yml and release.yml. It runs in the repo, so it targets the pinned
toolchain. release.yml needed it too: without this the real release build
would have failed at tag time on Windows and Intel Mac.

Verified locally: reproduced the missing target on the pinned toolchain,
applied the fix, then cargo check -p vestige-mcp --target x86_64-apple-darwin
with the full Intel Mac release feature set exits 0.

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

.github/workflows/ci.yml

@@ -96,6 +96,13 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
+      # rust-toolchain.toml pins the toolchain, so cargo runs the pinned version
+      # rather than the stable one the action installed. Add the cross-compile
+      # target to the pinned toolchain too, or the build fails with
+      # "can't find crate for `core`".
+      - name: Add target to the pinned toolchain
+        run: rustup target add ${{ matrix.target }}
+
       - name: Cache cargo
         uses: actions/cache@v4
         with:

.github/workflows/release.yml

@@ -75,6 +75,13 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
+      # rust-toolchain.toml pins the toolchain, so cargo runs the pinned version
+      # rather than the stable one the action installed. Add the cross-compile
+      # target to the pinned toolchain too, or the build fails with
+      # "can't find crate for `core`".
+      - name: Add target to the pinned toolchain
+        run: rustup target add ${{ matrix.target }}
+
       - name: Validate release version
         shell: bash
         env: