chore(node): upgrade actions and align runtime to Node 22

[serithemage]

What Changed

• upgraded GitHub Actions workflow dependencies from actions/checkout@v4 and actions/setup-node@v4 to their current v6 releases
• aligned CDK NodejsFunction bundling/runtime defaults to Node.js 22 in packages/cdk/lib/stacks/api-stack.ts
• declared engines.node >= 22.0.0 at the workspace root
• updated development and deployment docs to state Node.js 22+ as the project baseline

What Did Not Change

• no application feature behavior changed
• no Bedrock, runtime-routing, or maintainer-skill workflow logic changed in this PR
• no cloud resources were deployed from this branch

Security Impact

• Auth / identity changed: No
• Secrets / config loading changed: No
• Network / outbound behavior changed: No
• AWS permissions / infrastructure changed: No

Evidence

• npm run build
• npm run lint
• npm run test:e2e
• local validation confirmed the updated CDK Node runtime/bundle target still synthesizes correctly
• upstream action releases checked via gh release view --repo actions/checkout and gh release view --repo actions/setup-node

Human Verification

• Verified scenarios: CI workflow files updated, CDK Lambda runtime target updated to Node 22, docs and package metadata aligned
• Edge cases checked: Actions runner deprecation warning target, Lambda runtime parity with existing Lambda Agent Node 22 base image
• Not verified: full deploy workflow execution against AWS