fix(#300): centralize selector fallback, fix dry-run link discovery, and smart --config routing

- Add FALLBACK_MAIN_SELECTORS constant and _find_main_content() helper to
  eliminate 3 duplicated fallback loops in doc_scraper.py
- Move link extraction before early return in extract_content() so links
  are always discovered from the full page, not just main content
- Fix single-threaded dry-run to extract links from soup (full page)
  instead of main element only — fixes reactflow.dev finding only 1 page
- Add link extraction to async dry-run path (was completely missing)
- Remove main_content from get_configuration() defaults so fallback logic
  kicks in instead of a broad CSS comma selector matching body
- Smart create --config routing: peek at JSON to determine unified
  (sources array → unified_scraper) vs simple (base_url → doc_scraper)
- Update docs/user-guide/02-scraping.md and docs/reference/CONFIG_FORMAT.md
  to use unified config format (legacy format rejected since v2.11.0)
- Fix test_auto_fetch_enabled and test_mcp_validate_legacy_config

Closes #300

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

configs/react.json

@@ -0,0 +1,69 @@
+{
+  "name": "react",
+  "description": "Complete React knowledge base combining official documentation and React codebase insights. Use when working with React, understanding API changes, or debugging React internals.",
+  "version": "1.1.0",
+  "merge_mode": "rule-based",
+  "sources": [
+    {
+      "type": "documentation",
+      "base_url": "https://react.dev/",
+      "extract_api": true,
+      "selectors": {
+        "main_content": "article",
+        "title": "h1",
+        "code_blocks": "pre code"
+      },
+      "url_patterns": {
+        "include": [],
+        "exclude": [
+          "/blog/",
+          "/community/"
+        ]
+      },
+      "categories": {
+        "getting_started": [
+          "learn",
+          "installation",
+          "quick-start"
+        ],
+        "components": [
+          "components",
+          "props",
+          "state"
+        ],
+        "hooks": [
+          "hooks",
+          "usestate",
+          "useeffect",
+          "usecontext"
+        ],
+        "api": [
+          "api",
+          "reference"
+        ],
+        "advanced": [
+          "context",
+          "refs",
+          "portals",
+          "suspense"
+        ]
+      },
+      "rate_limit": 0.5
+    },
+    {
+      "type": "github",
+      "repo": "facebook/react",
+      "enable_codebase_analysis": true,
+      "code_analysis_depth": "deep",
+      "fetch_issues": true,
+      "max_issues": 100,
+      "fetch_changelog": true,
+      "fetch_releases": true,
+      "file_patterns": [
+        "packages/react/src/**/*.js",
+        "packages/react-dom/src/**/*.js"
+      ]
+    }
+  ],
+  "base_url": "https://react.dev/"
+}