skill: decide video length from the content, with no 60-second default #30

Problem

SKILL.md sets the default length to "about 60 seconds (6-9 scenes)". Nothing in the code limits the length, but in practice every video comes out around a minute, and a theme that needs more is cut to fit. The figure came from the Claude Motion era; the html path has no reason for it, and even with Motion a longer video should be possible.

Proposal

• SKILL.md: decide the length from the theme's content, with no upper limit. One mechanism: about 60 s; a theme with several steps or comparisons: a few minutes. A length the user gives always wins
• When a video will be longer than about 2 minutes, tell the user up front how long rendering takes (about 5 min per minute of video today, see perf(build): capture html film frames on several pages in parallel #25), without stopping to ask
• README (en / ja): say there is no upper limit and give the render time per minute

Check with a real video

Make one video of about 3 minutes end to end and confirm:

• BGM: Lyria is asked for the film length + 10 s. Whether it returns a long enough track is unverified; if it is short, the mix loops it, and the seam may be audible
• The html film: one page with many scenes. Watch for layout or quality problems as the page grows
• Render time against the baseline in perf(build): capture html film frames on several pages in parallel #25

Part of #16.