docs(prompting): document the intent interview and align pages with skill contracts

[WaterrrForever]

Stacked on #2109 — targets its branch, since the docs/prompting/ pages only exist there.

What

• Add "The interview: what the agent asks first" to the prompting overview: the route confirm, the must-have questions, the two run-shape questions (storyboard; automation vs companion) with the four-valid-combinations note, the "just build it" skip, and BRIEF.md as the artifact later sessions resume from.
• Add a note on the storyboards page separating the plan this page teaches from the interview's storyboard question — that answer changes the review process, not the route.
• Make BRIEF.md the pipeline guide's Step 3 artifact (step table, project tree, step body, gate, iterating list), and fix SCRIPT.md's mislabeled step in the tree (Step 4, not 3).
• Correct the workflow one-liners in the setup tables against each SKILL.md contract: general-video leads with its positive identity and companion mode; faceless-explainer keys on invented visuals instead of TTS; talking-head-recut uses its "graphic overlays" trigger term; motion-graphics gains an input side and its transparent-overlay output; music-to-video stops implying images are required. The quickstart mirror is updated in lockstep.
• Rework the vocabulary page's two video grids: 3-column CSS grid, autoplay muted loops, 16:9 alignment. The old 4-5-column table rendered as unreadable black boxes (dark first frames behind preload="metadata", player chrome covering ~250px-wide videos, ragged cell heights).
• Rename "companion media pipeline" to "sibling media pipeline" in media-and-audio — companion is now a reserved term.

Why

The intent layer (BRIEF.md, flow, storyboard) is the front door of every skills run, but the guide never prepared readers for it: no page explained the interview, companion vs automation, or the file that makes a brief resumable. The catalog one-liners had also drifted from the SKILL.md descriptions they compress — including one that inverted a contract (music-to-video requires zero user assets).

Test plan

• Manual testing performed — every edited page verified on a local mint dev: pages render, the new #the-interview-what-the-agent-asks-first anchor resolves from both cross-links, video grids play and align
• Documentation updated