Clean mainline repo evidence boundaries

[lishehao]

Summary

• Remove tracked root .DS_Store from the public mainline diff.
• Add a compact README evidence-status table separating mainline demo evidence, PR Add local mock user agent judge loop #18 Agent Trace/judge work, and Story Brief beta exploration.
• Clarify that Agent Trace / Step-Contract-Trajectory Judge / mock-user evaluation remains PR evidence until PR Add local mock user agent judge loop #18 is clean and merged.

Validation

• git diff --check
• Verified touched README links exist: docs/CURRENT_SYSTEM_MAP.md, docs/eval/EVAL_V3_REDESIGN.md

Boundary

• Docs/hygiene only.
• Does not include PR Add local mock user agent judge loop #18 Agent Trace/eval code.
• Does not include Story Brief v2 code.
• Does not claim PR Add local mock user agent judge loop #18 is merged, GitHub-CI-green, or fully validated.