feat(extension): add elastic extension command group (list, install, remove)

[margaretjgu]

Summary

Closes #296. Part of #222. Depends on #301.

• Adds src/extension/register.ts with registerExtensionCommands() building the elastic extension group
• Three subcommands: list, install <source>, remove <name>
• Extension commands bypass config loading (no Elastic cluster needed); handled by skipping the preAction hook for any command whose ancestor is extension
• cli.ts: lazy-loads registerExtensionCommands() when firstArg === 'extension'; excludes extension from the early config load path alongside config, sanitize, and version

Test plan

• npx tsc --noEmit passes clean
• elastic extension --help shows list/install/remove subcommands
• elastic extension list returns [] with no extensions installed
• elastic extension install github:elastic/elastic-local (functional, requires network)
• elastic extension remove local after install

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	6.4s
✅ REPOSITORY	gitleaks	yes		no	no	91.74s
✅ REPOSITORY	git_diff	yes		no	no	0.47s
✅ REPOSITORY	secretlint	yes		no	no	22.78s
✅ REPOSITORY	trivy	yes		no	no	16.2s
✅ TYPESCRIPT	eslint	6		0	0	3.39s

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