[Infra] Add release workflow and cosign public key

Add create-release.yml workflow triggered via workflow_dispatch to create
GitHub releases with auto-generated notes. Add cosign public key for
container image signature verification.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/create-release.yml

@@ -0,0 +1,60 @@
+name: Create Release
+
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: "Release tag (e.g. v1.83.0-stable)"
+        required: true
+        type: string
+      commit_hash:
+        description: "Full 40-char commit SHA to target"
+        required: true
+        type: string
+
+permissions: {}
+
+jobs:
+  release:
+    name: Create Release
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Validate inputs
+        env:
+          TAG: ${{ inputs.tag }}
+          COMMIT_HASH: ${{ inputs.commit_hash }}
+        run: |
+          if ! echo "${COMMIT_HASH}" | grep -qE '^[0-9a-f]{40}$'; then
+            echo "::error::commit_hash must be a full 40-character commit SHA"
+            exit 1
+          fi
+          if ! echo "${TAG}" | grep -qE '^v[0-9]+\.[0-9]+\.[0-9]+'; then
+            echo "::error::tag must start with vX.Y.Z"
+            exit 1
+          fi
+
+      - name: Create release
+        env:
+          TAG: ${{ inputs.tag }}
+          COMMIT_HASH: ${{ inputs.commit_hash }}
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        with:
+          script: |
+            const tag = process.env.TAG;
+            const commitHash = process.env.COMMIT_HASH;
+            try {
+              await github.rest.repos.createRelease({
+                draft: false,
+                generate_release_notes: true,
+                target_commitish: commitHash,
+                name: tag,
+                owner: context.repo.owner,
+                prerelease: false,
+                repo: context.repo.repo,
+                tag_name: tag,
+              });
+            } catch (error) {
+              core.setFailed(error.message);
+            }

cosign.pub

@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEKi4ivqGpE231OGH50PKbqy1Y1Kkb
+POJC8+i2Wko82gBOUCe3M0Vw86H/4rhUhfoYEti4gdJ9wZbYmK0I2EE96g==
+-----END PUBLIC KEY-----