fix: resolve following profile id without user-setting and fix testConnectivity (v0.3.16)

[cucoleadan]

Summary

• Resolves Substack returning HTTP 400 for PUT /user-setting with type: "last_home_tab".
• In getFollowing(options?: { profileId?: number | string }), allows passing profileId directly. If omitted, discovers the authenticated user ID via getAuthenticatedProfile() / /api/v1/handle/options instead of the broken PUT /user-setting.
• Fixes testConnectivity() so it tests a valid authenticated endpoint (GET /api/v1/handle/options) instead of PUT /user-setting.
• Adds getSubscriptions(options?: { handle?: string; profileId?: number | string }) for fetching publication subscriptions (authenticated account or public profile).
• Adds get_following and get_subscriptions tools to the STDIO MCP server.
• Updates documentation in README.md.
• Bumps package version to 0.3.16.

Validation

• Added or updated focused tests
• Ran bun run test:all
• Ran bun run build
• Updated public documentation when needed
• Confirmed no credentials, cookies, or private publication data are included