feat: restore PublisherAdapter interfaces and platform publishers

[stackbilt-admin]

Context

Publishing adapters were committed then reverted (3a4e370). They belong in OSS — generic platform clients (Bluesky/Twitter/Facebook API wrappers) are infrastructure, not competitive advantage.

What to restore

1. PublisherAdapter interface — mirrors PlatformAdapter for outbound operations
2. BlueskyPublisher — AT Protocol client (post, like, repost, reply, follow, feed, delete)
3. TwitterPublisher — X API v2 client (ties into existing social-sentinel#30)
4. FacebookPublisher — Graph API client

Constraints

• No D1 dependency — OSS version uses adapters only, no persistence
• No cron publisher — scheduling/orchestration is the managed layer's job (Social Hub)
• Must export clean TypeScript interfaces for downstream consumers
• PII redaction should be available as middleware on the publish path too

Why OSS

• Attracts contributors who add platform support (LinkedIn, Mastodon, Threads)
• Makes Social Sentinel useful as a standalone tool (monitoring-only is half a product)
• Establishes the adapter pattern that Social Hub extends
• Creates adoption funnel for the commercial managed hub

Related

• social-hub#3 — Social Hub will depend on this package instead of forking
• social-sentinel#30 — X/Twitter posting adapter (can be implemented as part of this)

[stackbilt-admin]

ODD Alignment (ref #42)

Publisher adapters are correctly at the implementation layer — ODD contracts don't own adapter internals. But they should implement against contract types:

• PublisherAdapter.publish() input should align with PostContract.operations.publish
• PublisherAdapter.engage() input should align with EngagementContract.operations.record
• Platform-specific constraints (rate limits, content length, grapheme counting) are codified in PLATFORM_RATE_LIMITS exported from PlatformContract

The PlatformCredentials discriminated union in platform.contract.ts defines per-platform credential shapes (Bluesky, Twitter, Facebook, LinkedIn, dev.to) — adapter implementations should validate against these schemas.