fix(ci): point Compose macOS signing at the CI keychain explicitly

The desktop DMG release leg (build-desktop macos, packageReleaseDmg) has never
produced a signed artifact: createReleaseDistributable fails with "Could not
find certificate for '***' in keychain []". This is independent of the v1.12.3
notarization fix, which addressed the separate amy CLI leg.

Root cause: Compose's MacSignerImpl maps the signing identity to a certificate
by running `security find-certificate -a -c <identity>` with no keychain
argument. On the GitHub macOS runners that lookup does not resolve the cert that
import-macos-cert imported into a throwaway keychain and added only to the user
search list — even though bare `codesign --sign` (e.g. the signMacJarNatives
task, which succeeds in the same job) finds it fine. The "keychain []" in the
error is just the null settings.keychain being echoed.

Fix: export the throwaway keychain path from the import-macos-cert action and
feed it to Compose's `signing.keychain` via AMETHYST_MAC_SIGN_KEYCHAIN, so the
certificate lookup searches that keychain directly. Also set it as the default
keychain for good measure. No-op on local/PR builds (env unset -> Compose keeps
its previous default-search-list behavior).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/actions/import-macos-cert/action.yml

@@ -17,6 +17,13 @@ outputs:
   signing:
     description: "'true' if a certificate was imported, else 'false'"
     value: ${{ steps.import.outputs.signing }}
+  keychain:
+    description: >
+      Absolute path of the throwaway keychain holding the imported cert (empty
+      when signing=false). Pass to tools that need an explicit keychain — e.g.
+      Compose's macOS signing, whose certificate lookup doesn't resolve the
+      search list reliably on CI runners the way bare `codesign` does.
+    value: ${{ steps.import.outputs.keychain }}
 
 runs:
   using: composite
@@ -48,5 +55,12 @@ runs:
         # Prepend our keychain to the user search list so codesign sees it.
         security list-keychains -d user -s "$KEYCHAIN" \
           $(security list-keychains -d user | sed -e 's/[\"[:space:]]//g')
+        # Also make it the default keychain. Bare `codesign` resolves identities
+        # via the search list, but some tools (Compose's MacSigner runs
+        # `security find-certificate` to map the identity to a cert) don't find
+        # it on the search list alone on these runners. Callers that need an
+        # explicit keychain can read the `keychain` output below.
+        security default-keychain -d user -s "$KEYCHAIN"
         rm -f "$CERT_PATH"
         echo "signing=true" >> "$GITHUB_OUTPUT"
+        echo "keychain=$KEYCHAIN" >> "$GITHUB_OUTPUT"