feat(docs): add CLI reference documentation generator

[ssh-esh]

To reviewers: This is a large PR. It's best to review by commit. The last commit adds all the generated files (Markdown docs + JSON schemas). All preceding commits contain the feature logic and tests.

Closes https://github.com/elastic/agentic-interface-program/issues/54

Adds automatic CLI reference documentation generation from the command tree.

• src/lib/doc-generator.ts: walks the Commander command tree and extracts structured metadata (options, args, aliases) into plain objects, with a Markdown renderer and JSON Schema extractor
• scripts/generate-docs.ts: orchestrates generation: builds the full program, extracts schemas, writes Markdown reference pages and per-command JSON Schema files to docs/reference/
• npm run docs:generate / npm run docs:check — generate docs or verify all commands have descriptions
• CI docs-check job: verifies docs generate without errors, committed docs are up-to-date (git diff), and all commands have descriptions

JSON Schema files are generated per leaf command (empty schemas filtered out per review feedback). Each command's Markdown section links to its schema file for machine consumption.

Follow-up: man page generation tracked in elastic/agentic-interface-program#83.

Test plan

• npm test passes (24 new tests for doc-generator)
• npm run docs:generate produces docs without errors
• npm run docs:check reports all commands have descriptions
• Regenerating docs produces no diff (freshness check)

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	1		0	0	0.11s
✅ COPYPASTE	jscpd	yes		no	no	5.21s
✅ REPOSITORY	gitleaks	yes		no	no	86.58s
✅ REPOSITORY	git_diff	yes		no	no	0.31s
✅ REPOSITORY	secretlint	yes		no	no	14.22s
✅ REPOSITORY	trivy	yes		no	no	24.42s
✅ TYPESCRIPT	eslint	3		0	0	4.49s
✅ YAML	yamllint	1		0	0	0.85s

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[JoshMock]

Closing in favor of #264