feat(skills): add research-synthesizer skill for multi-document analysis and synthesis

Defines triggers, input requirements, sub-agent usage patterns, and
output formats for synthesizing insights across multiple documents.
Covers standard research synthesis, vendor comparisons, multi-perspective
analyses, and web + local source combinations. Includes example prompts.

cowork-complete-guide/skills/research-synthesizer.md

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+# Research Synthesizer Skill
+
+## When to Use
+Use this skill whenever you need to extract meaning from multiple documents — not just summarize them individually, but find patterns, contradictions, and insights *across* the full set.
+
+Ideal triggers:
+- "Analyze these [N] documents and tell me what they say collectively"
+- "Research [topic] and give me a synthesis"
+- "What are the key themes across all of this?"
+- "Compare these sources and find contradictions"
+- "Give me strategic recommendations based on this research"
+
+---
+
+## Input Requirements
+
+**Minimum viable input:**
+- 2+ documents (files, URLs, or pasted content)
+- A clear question or goal ("what should we do about X?" or "what patterns emerge?")
+
+**Better inputs include:**
+- The decision or action this research will inform
+- The audience for the output (executive, team, personal reference)
+- Any specific angles or perspectives to prioritize
+
+**Supported input types:**
+- Local files (any format: .txt, .md, .pdf, .docx)
+- Folder paths ("analyze everything in /research/")
+- Web URLs (for current/public information)
+- Mixed local + web sources
+
+---
+
+## When to Request Sub-Agents
+
+Use parallel sub-agents when:
+- **5+ documents** — process batches simultaneously instead of sequentially
+- **Vendor/option comparison** — assign one agent per vendor so analyses are independent and unbiased
+- **Multi-perspective analysis** — assign one agent per perspective (financial, customer, operational) so each goes deep without anchoring on others
+- **Large corpus (20+)** — split into thematic batches, synthesize batch outputs into final report
+
+You can explicitly request this:
+> "Analyze these documents using parallel processing"
+> "Research each vendor independently, then compare"
+> "Analyze this decision from financial, customer, and operational perspectives simultaneously"
+
+---
+
+## Output Format
+
+### Standard Research Synthesis
+1. **Executive Summary** — 3–5 sentence answer to the core question
+2. **Key Themes** — dominant patterns across documents (with source citations)
+3. **Supporting Data** — statistics, quotes, and specific facts compiled in one place
+4. **Contradictions & Tensions** — where sources disagree or create trade-offs
+5. **Strategic Recommendations** — what to do, in priority order
+6. **Questions for Further Research** — what the documents don't answer
+
+### Vendor Comparison Output
+- Individual vendor profiles (price, performance, reliability, service, fit)
+- Side-by-side comparison table
+- Clear recommendation with rationale
+- Conditions under which the recommendation changes
+
+### Multi-Perspective Output
+- One section per perspective (each argued independently)
+- Synthesis section showing cross-perspective patterns
+- Single overall recommendation
+- Conditions/triggers that change the recommendation
+
+---
+
+## How to Incorporate Web Research
+
+Combine local files + web search for the most complete picture:
+
+- **Local files** = proprietary data, historical records, internal strategy, meeting notes
+- **Web search** = current market conditions, competitor moves, public data, recent news
+
+When to add web research:
+- Local files are more than 12 months old (market conditions may have shifted)
+- You need competitor or industry context not in internal documents
+- You want to pressure-test internal assumptions against external reality
+
+How to request it:
+> "Analyze my internal documents AND search for recent [topic] news to add external context"
+> "After synthesizing the local files, search for what competitors are doing on [topic]"
+
+---
+
+## Example Prompts
+
+**Large document set:**
+> "I have 40 customer interview transcripts in /research/interviews/. What are the top 5 themes about why customers churn? Use parallel processing and create a synthesis report."
+
+**Vendor comparison:**
+> "Compare these three CRM vendors — Salesforce, HubSpot, and Pipedrive — based on the files in /vendor-research/. Research each independently then give me a recommendation for a 10-person sales team."
+
+**Multi-perspective:**
+> "Should we launch in the European market? Analyze this decision from financial, legal/compliance, and customer adoption perspectives simultaneously. Then synthesize."
+
+**Local + web:**
+> "Read our internal competitive analysis from last year and add current web research on [Competitor X]. Tell me what's changed and update our positioning recommendations."