fix(ci): add input validation and allowlist enforcement for compute types

Addresses 5 security findings:

1. CRITICAL: deploy.yml wildcard case now validates intent against
   ALLOWED_COMPUTE_TYPES before passing to matrix. Invalid values
   cause the workflow to fail with an error annotation.

2. MEDIUM: PR label deploy:<type> values are filtered through
   validate_compute_type(). Invalid labels emit a warning and are
   ignored rather than passed to the deploy matrix.

3. MEDIUM: sanitize() now lowercases input and prefixes "s-" if the
   result starts with a digit (CloudFormation requires letter start).

4. LOW: deploy-intent.json is now written with jq (safe JSON encoding)
   instead of shell string interpolation.

5. LOW: PR_NUMBER is validated as numeric before use in stack names.

The ALLOWED_COMPUTE_TYPES allowlist is defined as an env var in each
step that performs validation. When new compute types are added to the
matrix, this allowlist must be updated in both build.yml and deploy.yml.

Part of #73 Phase 3.

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

Author: scottschreckengaust

.github/workflows/build.yml

@@ -32,8 +32,32 @@ jobs:
           GH_TOKEN: ${{ github.token }}
           REPO: ${{ github.repository }}
           PR_NUMBER_FROM_EVENT: ${{ github.event.workflow_run.pull_requests[0].number }}
+          ALLOWED_COMPUTE_TYPES: "agentcore"
         run: |
           ALL_TYPES='["agentcore"]'
+
+          validate_compute_type() {
+            local type="$1"
+            for allowed in $ALLOWED_COMPUTE_TYPES; do
+              [[ "$type" == "$allowed" ]] && return 0
+            done
+            echo "::error::Invalid compute_type: '$type'. Allowed: $ALLOWED_COMPUTE_TYPES"
+            return 1
+          }
+
+          filter_valid_types() {
+            local input_json="$1"
+            local valid_json="[]"
+            for type in $(echo "$input_json" | jq -r '.[]'); do
+              if validate_compute_type "$type" 2>/dev/null; then
+                valid_json=$(echo "$valid_json" | jq --arg t "$type" '. + [$t]')
+              else
+                echo "::warning::Ignoring invalid compute_type from label: '$type'"
+              fi
+            done
+            echo "$valid_json"
+          }
+
           INTENT=$(jq -r '.deploy' deploy-intent.json)
           echo "Deploy intent from build: $INTENT"
 
@@ -43,7 +67,6 @@ jobs:
               echo "has_targets=false" >> "$GITHUB_OUTPUT"
               ;;
             labels)
-              # PR-triggered build — check labels
               if [[ -z "$PR_NUMBER_FROM_EVENT" ]]; then
                 echo "matrix=[]" >> "$GITHUB_OUTPUT"
                 echo "has_targets=false" >> "$GITHUB_OUTPUT"
@@ -56,9 +79,17 @@ jobs:
                 echo "matrix=$ALL_TYPES" >> "$GITHUB_OUTPUT"
                 echo "has_targets=true" >> "$GITHUB_OUTPUT"
               elif echo "$LABELS" | jq -e '[.[] | select(startswith("deploy:"))] | length > 0' > /dev/null 2>&1; then
-                TYPES=$(echo "$LABELS" | jq -c '[.[] | select(startswith("deploy:")) | ltrimstr("deploy:")]')
-                echo "matrix=$TYPES" >> "$GITHUB_OUTPUT"
-                echo "has_targets=true" >> "$GITHUB_OUTPUT"
+                RAW_TYPES=$(echo "$LABELS" | jq -c '[.[] | select(startswith("deploy:")) | ltrimstr("deploy:")]')
+                VALIDATED=$(filter_valid_types "$RAW_TYPES")
+                COUNT=$(echo "$VALIDATED" | jq 'length')
+                if [[ "$COUNT" -gt 0 ]]; then
+                  echo "matrix=$VALIDATED" >> "$GITHUB_OUTPUT"
+                  echo "has_targets=true" >> "$GITHUB_OUTPUT"
+                else
+                  echo "::warning::All deploy:<type> labels were invalid"
+                  echo "matrix=[]" >> "$GITHUB_OUTPUT"
+                  echo "has_targets=false" >> "$GITHUB_OUTPUT"
+                fi
               elif echo "$LABELS" | jq -e 'index("deploy")' > /dev/null 2>&1; then
                 echo "matrix=$ALL_TYPES" >> "$GITHUB_OUTPUT"
                 echo "has_targets=true" >> "$GITHUB_OUTPUT"
@@ -68,7 +99,11 @@ jobs:
               fi
               ;;
             *)
-              # Specific compute_type from push-to-main or workflow_dispatch
+              if ! validate_compute_type "$INTENT"; then
+                echo "matrix=[]" >> "$GITHUB_OUTPUT"
+                echo "has_targets=false" >> "$GITHUB_OUTPUT"
+                exit 1
+              fi
               echo "matrix=[\"$INTENT\"]" >> "$GITHUB_OUTPUT"
               echo "has_targets=true" >> "$GITHUB_OUTPUT"
               ;;