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/commands/reference-discovery.md

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+---
+description: Search and discover authoritative reference sources for a topic. Validates credibility, generates URL manifests for crawling.
+argument-hint: <topic> [--vendor anthropic|openai|google] [--max-sources 10]
+allowed-tools: WebSearch, WebFetch, Read, Write, Bash, Grep, Glob
+---
+
+# Reference Discovery
+
+Search for authoritative reference sources on a given topic.
+
+## Arguments
+- `<topic>`: Required. The subject to find references for (e.g., "Claude system prompts")
+- `--vendor`: Filter to specific vendor (anthropic, openai, google)
+- `--max-sources`: Maximum sources to discover (default: 10)
+
+## Workflow
+
+### 1. Search Strategy
+Use multiple search approaches:
+- Official documentation sites
+- Engineering blogs
+- GitHub repositories
+- Research papers
+- Community guides
+
+### 2. Source Validation
+
+Evaluate each source for credibility:
+
+| Tier | Description | Examples |
+|------|-------------|----------|
+| tier1_official | Vendor documentation | docs.anthropic.com |
+| tier2_verified | Verified engineering blogs | anthropic.com/news |
+| tier3_community | Community resources | GitHub repos, tutorials |
+
+### 3. Output Manifest
+
+Generate `manifest.json` in working directory:
+
+```json
+{
+  "topic": "user provided topic",
+  "discovered_at": "ISO timestamp",
+  "sources": [
+    {
+      "url": "https://...",
+      "title": "Page title",
+      "source_type": "official_docs",
+      "credibility_tier": "tier1_official",
+      "vendor": "anthropic"
+    }
+  ]
+}
+```
+
+### 4. Store Sources
+
+Insert discovered sources into MySQL:
+```bash
+source ~/.envrc
+mysql -u $MYSQL_USER -p"$MYSQL_PASSWORD" reference_library
+```
+
+Use the `sources` table schema from `~/.config/reference-curator/`.
+
+## Example Usage
+
+```
+/reference-discovery Claude's system prompt best practices
+/reference-discovery MCP server development --vendor anthropic
+/reference-discovery prompt engineering --max-sources 20
+```