fix: harden real README source-to-brief intake

[1beenlee]

Summary

Public README trials exposed source-compression problems before full deck generation:

• required_links could become a dump of every setup/dependency/reference URL even though the schema means links that must actually appear in the presentation
• Markdown/HTML image references could be mistaken for supplied image assets in a text-only workflow
• lifecycle caveats such as sunset/archived/deprecated status could be lost while compressing a README into a cleaner story
• qualified source claims could be strengthened accidentally during summarization

This PR hardens the source-to-brief contract without adding new fields or ingestion scope:

• select presentation-essential links only
• treat referenced images as unavailable unless their actual assets were separately supplied; presentation-critical missing images become missing_information
• preserve material lifecycle/status caveats
• preserve quantitative qualifiers such as about, under, and up to
• document the same semantics in the brief schema and Agent workflow
• lock the rules into tools/test-agent-workflow.mjs, including the staged source-to-brief packet

Scope guardrails

• no third-party README or derived real-project trial artifact is committed
• no schema field names/shapes changed
• no URL/image/repository fetching, model/provider dependency, deploy, Preview, or hosted product work
• examples/, templates/, .agents/, .claude/ unchanged

Validation target

Existing CI must remain green. The Agent workflow regression must explicitly prove the new prompt contract is present in both the canonical prompt and staged source-to-brief packet.

Draft until exact-head CI and diff review pass.