polish: S3 prompts — add topic context and niche awareness

[0b00101111]

Problem

S3 generates scripts in a vacuum — no niche, no audience, no topic constraints. Output is generic ("phone battery hack", "bad morning story") because the prompt has no context about what the creator's channel is about or who watches it.

This is the biggest quality gap in the pipeline.

Suggested Improvements

1. Add niche and target_audience to the generation prompt (from creator profile)
2. Pass 2-3 verbatim hook examples from S2 as few-shot references
3. Add topic selection as an explicit step — from creator input or inferred from past content
4. Add "specificity score" self-evaluation — force the LLM to rate how generic its output is
5. Generate hooks independently first, then build scripts around the best hooks (hook quality is the Add spec v2 design document #1 driver of views)

Context

See design/pipeline-quality-review.md — S3 is listed as priority #1 for quality improvements.

Depends on

• PR feat: dataset prep + first real pipeline run (M1-6) #49 (M1-6 dataset + pipeline run)
• CreatorProfile expansion (polish: expand CreatorProfile model #52 when created)