feat(extension): add search and upgrade commands

[margaretjgu]

Summary

Closes #298. Part of #222. Depends on #303.

elastic extension search [query]

• Queries the GitHub repository search API for repos tagged with elastic-extension topic
• Optional free-text query to narrow results
• Returns repo, description, url, and a ready-to-paste installCommand for each result
• Handles rate-limit (403/429) with a clear message suggesting GITHUB_TOKEN
• fetch() called with redirect: 'error' to prevent credential leakage on redirects
• New module src/extension/search.ts

elastic extension upgrade [name]

• With a name: upgrades one extension (git pull --ff-only for github, npm update for npm), rediscovers entrypoint, updates registry
• Without a name: upgrades all installed extensions; collects per-extension errors and reports them together rather than stopping on first failure
• Added to src/extension/installer.ts alongside the existing install/uninstall functions

Test plan

• npx tsc --noEmit passes clean
• 16/16 tests pass (upgradeExtension error path, upgradeAllExtensions empty case, 6 search stub tests)
• ~/.bun/bin/bun test test/extension/ -- all pass
• elastic extension search (functional, requires network)
• elastic extension upgrade local after install (functional, requires git)

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	7.12s
✅ REPOSITORY	gitleaks	yes		no	no	56.42s
✅ REPOSITORY	git_diff	yes		no	no	0.54s
✅ REPOSITORY	secretlint	yes		no	no	29.6s
✅ REPOSITORY	trivy	yes		no	no	19.01s
✅ TYPESCRIPT	eslint	10		0	0	5.39s

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