Add a staging environment for Ask Mantosh Worker changes

Problem

docs/SYSTEM_STATE.md explicitly notes that the repository has no declared staging environment. Worker deployment is separate from GitHub Pages deployment, so meaningful runtime changes currently lack a documented pre-production validation target.

Goal

Create the smallest useful staging path for Ask Mantosh without turning the personal website into a heavy deployment platform.

Scope

• Define a staging Worker environment with isolated configuration.
• Keep production credentials, D1 data, Vectorize data, quotas, and cache state separated where required.
• Document deployment and rollback steps.
• Add a staging smoke check covering health, deterministic navigation, one grounded retrieval path, and a failure path.
• Preserve the production exact-origin/CORS security model.

Acceptance criteria

• A documented staging Worker can be deployed independently of production.
• Staging cannot mutate production knowledge or production runtime state.
• A deterministic smoke test validates the staged deployment.
• Production deployment instructions identify when staging verification is required.
• docs/SYSTEM_STATE.md and relevant release documentation are updated.

Start with docs/SYSTEM_STATE.md and the existing production deployment instructions, then map the Worker’s configuration and state boundaries. Done means an independently deployable staging Worker, documented deployment and rollback steps, a deterministic smoke check for the listed paths, and updated system-state and release documentation.