feat: Add YouTube as 4th research source via yt-dlp

YouTube search and transcript extraction runs automatically when yt-dlp
is installed. Searches for topic videos from the last N days, fetches
auto-generated transcripts for top results, and feeds them through the
same scoring pipeline (relevance + recency + engagement) as Reddit/X.

New files:
- youtube_yt.py: search, transcript extraction, VTT cleanup

Modified files:
- schema.py: YouTubeItem dataclass, updated Report
- normalize.py: normalize_youtube_items()
- score.py: YouTube engagement scoring (views-dominated)
- dedupe.py: YouTube deduplication
- render.py: YouTube section in compact output
- env.py: is_ytdlp_available() check
- ui.py: YouTube progress messages
- last30days.py: _search_youtube(), parallel execution with Reddit/X
- SKILL.md: YouTube in stats box, citation priority
- README.md: YouTube docs, yt-dlp requirement, Peter shoutout

Inspired by Peter Steinberger's yt-dlp + summarize toolchain approach.

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

scripts/lib/ui.py

@@ -64,6 +64,14 @@ ENRICHING_MESSAGES = [
     "Analyzing discussions...",
 ]
 
+YOUTUBE_MESSAGES = [
+    "Searching YouTube for videos...",
+    "Finding relevant video content...",
+    "Scanning YouTube channels...",
+    "Discovering video discussions...",
+    "Fetching transcripts...",
+]
+
 PROCESSING_MESSAGES = [
     "Crunching the data...",
     "Scoring and ranking...",
@@ -280,6 +288,15 @@ class ProgressDisplay:
         if self.spinner:
             self.spinner.stop(f"{Colors.CYAN}X{Colors.RESET} Found {count} posts")
 
+    def start_youtube(self):
+        msg = random.choice(YOUTUBE_MESSAGES)
+        self.spinner = Spinner(f"{Colors.RED}YouTube{Colors.RESET} {msg}", Colors.RED)
+        self.spinner.start()
+
+    def end_youtube(self, count: int):
+        if self.spinner:
+            self.spinner.stop(f"{Colors.RED}YouTube{Colors.RESET} Found {count} videos")
+
     def start_processing(self):
         msg = random.choice(PROCESSING_MESSAGES)
         self.spinner = Spinner(f"{Colors.PURPLE}Processing{Colors.RESET} {msg}", Colors.PURPLE)
@@ -289,15 +306,21 @@ class ProgressDisplay:
         if self.spinner:
             self.spinner.stop()
 
-    def show_complete(self, reddit_count: int, x_count: int):
+    def show_complete(self, reddit_count: int, x_count: int, youtube_count: int = 0):
         elapsed = time.time() - self.start_time
         if IS_TTY:
             sys.stderr.write(f"\n{Colors.GREEN}{Colors.BOLD}✓ Research complete{Colors.RESET} ")
             sys.stderr.write(f"{Colors.DIM}({elapsed:.1f}s){Colors.RESET}\n")
             sys.stderr.write(f"  {Colors.YELLOW}Reddit:{Colors.RESET} {reddit_count} threads  ")
-            sys.stderr.write(f"{Colors.CYAN}X:{Colors.RESET} {x_count} posts\n\n")
+            sys.stderr.write(f"{Colors.CYAN}X:{Colors.RESET} {x_count} posts")
+            if youtube_count:
+                sys.stderr.write(f"  {Colors.RED}YouTube:{Colors.RESET} {youtube_count} videos")
+            sys.stderr.write("\n\n")
         else:
-            sys.stderr.write(f"✓ Research complete ({elapsed:.1f}s) - Reddit: {reddit_count} threads, X: {x_count} posts\n")
+            parts = [f"Reddit: {reddit_count} threads", f"X: {x_count} posts"]
+            if youtube_count:
+                parts.append(f"YouTube: {youtube_count} videos")
+            sys.stderr.write(f"✓ Research complete ({elapsed:.1f}s) - {', '.join(parts)}\n")
         sys.stderr.flush()
 
     def show_cached(self, age_hours: float = None):