docs: add Phase 1-3 PRD collection (12 documents)

[jessie-coco]

Summary

Add all 12 PRD documents to docs/prd/ covering ClawFeed Phase 1-3 feature specifications.

Phase 1 — Core Enhancement

• 1.2 个性化 Digest 生成 (Personalized Digest Generation)
• 1.3 Cron 采集集成 (Cron Collection Integration)
• 1.4 Sources 页面重构 (Sources Page Refactor)

Phase 2 — Intelligence Layer

• 2.1 多渠道推送 (Multi-Channel Push)
• 2.2 AI 互动助理 (AI Interactive Assistant)
• 2.3 Mark 增强 (Mark Enhancement)
• 2.4 Digest 质量提升 (Digest Quality Improvement)

Phase 3 — Platform Evolution

• 3.1 Agent Friendly API
• 3.2 Source Market
• 3.3 多语言 Digest (Multilingual Digest)
• 3.4 订阅组合缓存 (Subscription Combo Cache)
• 3.5 分析仪表板 (Analytics Dashboard)

Review Status

All 12 PRDs have been reviewed and approved by Lisa through the doc-discuss review process. Each document went through structured review with feedback incorporated.

Test Plan

• All 12 markdown files present in docs/prd/
• File naming follows clawfeed-prd-{phase}.{feature}.md convention
• Docs-only change — no code impact, no CI failures expected

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