docs(plan): LLM asset proposer implementation plan (4 TDD tasks)

Bite-sized TDD tasks: (1) proposal schema parsing, (2) screened-report builder
over an injected gate, (3) human+JSON renderers, (4) the assets propose CLI wired
to the real _screen_product. keel/proposer.py (pure) + thin cli.py command.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: eaitbrahim