feat(podcasts): add YouTube podcast source with transcript-first discovery

New "podcasts" source that discovers podcast content by scanning transcripts
from LLM-resolved YouTube channels. Finds content invisible to title-based
search — Acquired's "The NFL" episode mentions Taylor Swift 18x, ESPN 117x,
Netflix 102x, none in the title.

Architecture:
- LLM resolves 6-12 podcast channel @handles per topic
- Engine fetches recent episodes via yt-dlp (no video download)
- Downloads auto-captions and greps for topic keywords
- Episodes with 5+ mentions become podcast results with highlights
- Runs in parallel, ~15-20s latency, invisible in 3-min research run

Pipeline integration:
- New source module: scripts/lib/podcast_yt.py
- Registered in pipeline, normalizer, signals, planner, render
- CLI flag: --podcast-channels=AcquiredFM,lexfridman,...
- SOURCE_QUALITY: 0.88 (above YouTube's 0.85)
- Opt-in via INCLUDE_SOURCES=podcasts or --search=podcasts

Zero new API keys. Zero new dependencies. Reuses yt-dlp + transcript pipeline.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/last30days.py

@@ -170,6 +170,7 @@ def build_parser() -> argparse.ArgumentParser:
                         help="Use web search to discover subreddits/handles before planning (for platforms without WebSearch)")
     parser.add_argument("--github-user", help="GitHub username for person-mode search (e.g., steipete)")
     parser.add_argument("--github-repo", help="Comma-separated owner/repo for project-mode search (e.g., openclaw/openclaw,paperclipai/paperclip)")
+    parser.add_argument("--podcast-channels", help="Comma-separated YouTube @handles for podcast transcript scanning (e.g., AcquiredFM,lexfridman,DwarkeshPatel)")
     return parser
 
 
@@ -308,6 +309,7 @@ def main() -> int:
 
         github_user = args.github_user.lstrip("@").lower() if args.github_user else None
         github_repos = [r.strip() for r in args.github_repo.split(",") if r.strip() and "/" in r.strip()] if args.github_repo else None
+        podcast_channels = [c.strip().lstrip("@") for c in args.podcast_channels.split(",") if c.strip()] if args.podcast_channels else None
 
         # --deep-research: auto-enable perplexity source and set deep flag
         if args.deep_research:
@@ -337,6 +339,7 @@ def main() -> int:
             lookback_days=args.lookback_days,
             github_user=github_user,
             github_repos=github_repos,
+            podcast_channels=podcast_channels,
         )
     except Exception as exc:
         progress.end_processing()