feat(collectors): add optional auth-backed collectors without new tools

[endgegnerbert-tech]

Summary

Add optional cookie/auth-backed collectors after the no-auth collector system and consolidated emet integration are stable.

This remains separate because it has higher maintenance, privacy, credential, and platform-policy risk.

Candidate collectors

• Twitter/X via an explicitly installed local CLI or approved API path
• Reddit via an explicitly installed local CLI or approved API path
• Bilibili via yt-dlp where supported
• Weibo via safe reader/API path if available
• Douyin only through an explicitly configured local integration

Public surface

Do not add new MCP tools for each platform. Optional collectors should plug into the same internal collector registry from #18 and the same existing emet options flow from #19.

The agent-facing surface remains:

1. emet
2. web_fetch

Best-practice constraints

• Read-only only.
• No bundled credentials.
• No OAuth flow in core unless separately designed and reviewed.
• Cookie/CLI collectors must default to unavailable and include setup hints.
• User consent and local configuration must be explicit.
• Never send tokens/cookies to telemetry or logs.
• Respect platform terms, rate limits, and removal/block behavior.

Acceptance

• Optional collectors are invisible or clearly unavailable until configured.
• Doctor output explains required local setup per collector.
• Missing tools never crash MCP/Pi startup.
• Tests cover unavailable state and one mocked configured collector.
• tools/list does not gain platform-specific tools.

Out of scope

• Posting/writing.
• Headless browser automation.
• Credential management UI.
• Private account indexing.

Split from #17. Depends on #18 and #19.