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>

src/skill_seekers/cli/create_command.py

@@ -603,9 +603,30 @@ Common Workflows:
     log_level = logging.DEBUG if args.verbose else (logging.WARNING if args.quiet else logging.INFO)
     logging.basicConfig(level=log_level, format="%(levelname)s: %(message)s")
 
-    # Validate source provided
-    if not args.source:
-        parser.error("source is required")
+    # Validate source provided (config file can serve as source)
+    if not args.source and not args.config:
+        parser.error("source is required (or use --config to specify a config file)")
+
+    # If config is provided but no source, peek at the JSON to route correctly
+    if not args.source and args.config:
+        import json
+
+        try:
+            with open(args.config) as f:
+                config_peek = json.load(f)
+            if "sources" in config_peek:
+                # Unified format → route to unified_scraper via config type detection
+                args.source = args.config
+            elif "base_url" in config_peek:
+                # Simple web config → route to doc_scraper by using the base_url
+                args.source = config_peek["base_url"]
+                # source will be detected as web URL; --config is already set
+            else:
+                parser.error("Config file must contain 'sources' (unified) or 'base_url' (web)")
+        except json.JSONDecodeError as e:
+            parser.error(f"Cannot parse config file as JSON: {e}")
+        except FileNotFoundError:
+            parser.error(f"Config file not found: {args.config}")
 
     # Execute create command
     command = CreateCommand(args)