feat(reference-curator): Add portable skill suite for reference documentation curation

6 modular skills for curating, processing, and exporting reference docs:
- reference-discovery: Search and validate authoritative sources
- web-crawler-orchestrator: Multi-backend crawling (Firecrawl/Node/aiohttp/Scrapy)
- content-repository: MySQL storage with version tracking
- content-distiller: Summarization and key concept extraction
- quality-reviewer: QA loop with approve/refactor/research routing
- markdown-exporter: Structured output for Claude Projects or fine-tuning

Cross-machine installation support:
- Environment-based config (~/.reference-curator.env)
- Commands tracked in repo, symlinked during install
- install.sh with --minimal, --check, --uninstall modes
- Firecrawl MCP as default (always available)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

custom-skills/90-reference-curator/01-reference-discovery/code/CLAUDE.md

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+# Reference Discovery
+
+Search and identify authoritative sources for reference materials. Validates source credibility, prioritizes by relevance, and outputs curated URL lists with metadata.
+
+## Trigger Keywords
+"find references", "search documentation", "discover sources", "find authoritative materials", "research topic sources"
+
+## Source Priority Hierarchy
+
+| Tier | Source Type | Examples |
+|------|-------------|----------|
+| **Tier 1** | Official documentation | docs.anthropic.com, docs.claude.com, platform.openai.com/docs |
+| **Tier 1** | Engineering blogs (official) | anthropic.com/news, openai.com/blog |
+| **Tier 1** | Official GitHub repos | github.com/anthropics/*, github.com/openai/* |
+| **Tier 2** | Research papers | arxiv.org, papers with citations |
+| **Tier 2** | Verified community guides | Cookbook examples, official tutorials |
+| **Tier 3** | Community content | Blog posts, tutorials, Stack Overflow |
+
+## Workflow
+
+### Step 1: Define Search Scope
+Gather topic, target vendors, and freshness requirements from user input.
+
+### Step 2: Execute Web Search
+Use WebSearch tool with targeted queries:
+```
+site:docs.anthropic.com {topic}
+site:github.com/anthropics {topic}
+site:arxiv.org {topic}
+```
+
+### Step 3: Score and Validate Sources
+Apply credibility scoring:
+- Domain credibility (0.10 - 0.40)
+- Freshness signals (0.10 - 0.20)
+- Relevance signals (0.15)
+
+### Step 4: Output URL Manifest
+Generate JSON manifest for the crawler skill:
+
+```json
+{
+  "discovery_date": "2025-01-28T10:30:00",
+  "topic": "prompt engineering",
+  "total_urls": 15,
+  "urls": [
+    {
+      "url": "https://docs.anthropic.com/en/docs/prompt-engineering",
+      "title": "Prompt Engineering Guide",
+      "credibility_tier": "tier1_official",
+      "credibility_score": 0.85,
+      "source_type": "official_docs",
+      "vendor": "anthropic"
+    }
+  ]
+}
+```
+
+## Scripts
+
+### `discover_sources.py`
+Main discovery script. Usage:
+```bash
+python scripts/discover_sources.py --topic "prompt engineering" --vendors anthropic,openai --output manifest.json
+```
+
+## Output
+- `manifest.json` → Handoff to `02-web-crawler-orchestrator`
+- Register new sources in `sources` table via `03-content-repository`
+
+## Deduplication
+Before outputting:
+- Normalize URLs (remove trailing slashes, query params)
+- Check against existing `documents` table
+- Merge duplicates, keeping highest credibility score