Video Pack build planning: interview and evidence-cited plan #2504

Parent objective

This is the interview and planning slice of #2429. That parent issue remains open for real target-repository code operations, full build/test validation, and deployment evidence. This issue is tracked separately so a planning PR can close only the outcome it actually delivers.

Acceptance criteria

• A request to plan a build from a video requires a ready Video Pack whose canonical video and pack identities match; missing, processing, error, and identity-only packs fail closed.
• The hosted Studio and App Builder chat clients show the use-case interview questions, collect the answers, and preserve planning intent and pack identity on the follow-up turn.
• A completed interview returns a plan that separates user-supplied answers from cited Video Pack excerpts and marks unsupported frontend, backend, middleware, database, LSP, MCP, tool, and environment choices as unknown.
• Normal non-planning pack chat remains grounded in the existing model path, and an unbound request cannot emit a deterministic video-derived plan.
• Focused route/client tests and the affected project build/test checks pass. The PR description states what is still unverified in a live codebase.

Scope

Implementation PR: #2431. This issue does not cover autonomous target-repository implementation or deployment from #2429. The separate Sparkle template-slot objective in #2428 is also outside this issue.