v1.8.0 — scene-detection screenshot mode

New screenshot mode: ffmpeg scene detection — capture a frame whenever the
screen changes (slide flips, screen shares, demo cuts). Built for tutorial
videos, where slides change far more often than chapter markers suggest.

⚠ BREAKING

• Bare --screenshots now means scene detection, not chapter markers. Use
  --screenshots chapters to restore the pre-1.8.0 behavior. The legacy
  internal value auto is kept as an alias for chapters, so direct script
  callers passing --screenshots auto are unaffected.

yt-extract.py (backend)

Added

• Two-pass scene detection: pass 1 decodes a ≤360p stream once with ffmpeg's
  select='gt(scene,T)' filter (default threshold 0.025) and collects only
  timestamps; pass 2 extracts those frames at ≤1080p through the existing
  extract_screenshots() path. Rendering, filenames, and markdown embedding
  are unchanged (scene runs use the inline-with-heading layout).
• Flood protection: 4s minimum gap between detections, then even thinning to
  max 50 screenshots with a WARNING in ### Screenshot Status (suggests a
  higher threshold). Zero detections above threshold → opening frame only +
  WARNING suggesting a lower threshold.
• --screenshots scenes=0.05 — tunable detection threshold in (0, 1];
  invalid values fall back to the default with a WARNING.
• New stage marker Detecting scene changes (stderr, [k/N] stays adaptive).
  Detection decodes the whole video at low resolution and gets a
  duration-scaled timeout (min 5, max 30 minutes) plus HTTP reconnect flags.
• New pure helpers (unit-tested): parse_screenshots_mode,
  parse_scene_timestamps, apply_min_gap, thin_evenly; new subprocess
  helpers get_lowres_stream_url, detect_scene_timestamps.

Fixed

• Screenshot seeks no longer truncate timestamps to whole seconds
  (-ss int(ts) → -ss ts:.2f). For scene mode this is essential — a
  truncated seek could land before the detected change and capture the
  previous screen. Side effect: explicit fractional timestamps like 1:30.5
  now seek accurately instead of silently flooring to 1:30.

yt-extract skill

Changed

• Step 0.4 flag grammar: --screenshots [scenes[=t]|chapters|timestamps];
  both subagent prompts pass the user's value through verbatim and announce
  the new detection stage.
• SCREENSHOTS_ASK_USER now only fires for --screenshots chapters on a
  chapterless video — scene mode (the default) needs no chapters, so the
  no-chapters Rückfrage disappears from the happy path.
• No new sentinel: thinning/zero-detection conditions travel as - WARNING:
  lines inside ### Screenshot Status.

Tests

Added

• tests/test_scene_detection.py — pure-Python coverage for the four new
  helpers, including an ffmpeg metadata=print fixture verified against real
  output (pts_time: values may lack a decimal part).

Docs

Changed

• Version bumped 1.7.0 → 1.8.0 across CLAUDE.md, README.md,
  .claude-plugin/plugin.json, and .claude-plugin/marketplace.json;
  README gets a breaking-change callout and a scene-mode row in the flags
  and modes tables.