feat(exesh): send internal auth with heartbeats

[DIvanCode]

Closes#324

Exesh workers now read InternalAuthKey and send it with every coordinator heartbeat. Local and end-to-end worker/coordinator configurations use INTERNAL_AUTH_KEY_LOCAL_VALUE; the deployment playbook passes one encrypted internal_auth_key to the coordinator and both workers and validates it before deployment work begins. The heartbeat process documentation records the authenticated request.

Operator action before Ready for review: add the same non-local internal_auth_key to the encrypted credentials used by Duely, Taski, Exesh, and later Analyzer. Do not commit the plaintext value.

Validation:

• go test ./...
• ./e2e/taski-exesh/run.sh — accepted A+B solution
• ansible-playbook --syntax-check with temporary non-secret test variables