feat(extension): add context env-var propagation for extensions

[margaretjgu]

Summary

Closes #294. Part of #222.

• Adds src/extension/context.ts with a single buildContextEnv(config: ResolvedConfig) export
• Converts the active resolved config into a flat Record<string, string> to be merged into a child process env when spawning extensions
• Exposed variables (only set when the value is present):
  • ELASTIC_ES_URL, ELASTIC_ES_API_KEY / ELASTIC_ES_USERNAME + ELASTIC_ES_PASSWORD
  • ELASTIC_KIBANA_URL, ELASTIC_KIBANA_API_KEY / ELASTIC_KIBANA_USERNAME + ELASTIC_KIBANA_PASSWORD
  • ELASTIC_CLOUD_URL, ELASTIC_CLOUD_API_KEY
• 9 unit tests covering no-services, api_key auth, basic auth, auth-absent, each service independently, all three together, and no-undefined values guarantee

Test plan

• npx tsc --noEmit passes clean
• node --import tsx/esm --test test/extension/context.test.ts -- 9/9 pass

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	5.38s
✅ REPOSITORY	gitleaks	yes		no	no	90.49s
✅ REPOSITORY	git_diff	yes		no	no	0.44s
✅ REPOSITORY	secretlint	yes		no	no	25.52s
✅ REPOSITORY	trivy	yes		no	no	13.14s
✅ TYPESCRIPT	eslint	2		0	0	3.73s

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