docs: add daily brief implementation notes — context-from chaining, cost breakdown, prev state

hermes-automation/notes/daily-brief-implementation.md

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+# Daily Brief — Implementation Notes
+
+## Architecture: context-from Chaining
+
+The daily brief uses a **no-agent script → agent curation** pipeline:
+
+```
+┌──────────────────────────────┐
+│  cron: 0 15 * * *            │
+│  script: collector.sh        │  ← no-agent, free public APIs
+├──────────────────────────────┤
+│  Script stdout → agent       │  ← injected as prompt context
+│  context                     │
+├──────────────────────────────┤
+│  Agent: curates → formats    │  ← lightweight LLM pass only
+│  → delivers to Telegram      │
+└──────────────────────────────┘
+```
+
+## Cost Breakdown
+
+| Stage | Token Cost | Frequency |
+|-------|-----------|-----------|
+| Script fetch (shell) | $0 | 1x per tick |
+| Agent curation | Full model | 1x per tick |
+| **Total vs old approach** | **~90% cheaper** | Old: agent had to web search + X search (both failing) |
+
+## Sources (Free, No API Key)
+
+| Source | Endpoint | Reliability |
+|--------|----------|-------------|
+| Weather | `wttr.in/Placerville,CA` | 99.9% |
+| Hacker News | Firebase API (v0) | 99.9% |
+| GitHub Trending | `api.github.com/search/repositories` | 99% (unauthed rate limit: 60/hr — fine for daily) |
+
+## Previous State (Before June 20)
+
+The old prompt instructed the agent to use web_search and x_search tools to gather data. Both were failing silently — Firecrawl was unconfigured and xAI credits were exhausted. The brief was returning `[SILENT]` every run (zero useful output for weeks).
+
+## Key Insight
+
+The **script + context_from** pattern from the masterclass turned a dead cron job into a working one in <5 minutes, with zero ongoing API costs. The script does the heavy lifting (API calls, parsing), and the agent only does the lightweight curation pass.