feat(extension): context env-var propagation to extensions

[margaretjgu]

Part of #222.

When an extension is invoked, expose the active resolved config as environment variables so extensions can connect to Elasticsearch/Kibana without re-implementing config parsing.

• Module at src/extension/context.ts with a buildContextEnv(config: ResolvedConfig) export
• Env vars to set (only when values are present):
  • ELASTIC_ES_URL, ELASTIC_ES_API_KEY (or ELASTIC_ES_USERNAME/ELASTIC_ES_PASSWORD for basic auth)
  • ELASTIC_KIBANA_URL, ELASTIC_KIBANA_API_KEY
  • ELASTIC_CLOUD_API_KEY
• Unit tests in test/extension/context.test.ts

Depends on: #293