chore: prepare v0.2.8 release

[Grivn]

Summary

• bump dsh-mnemon to 0.2.8
• add English and Simplified Chinese release notes for bounded recall quality
• update README release badges and release-note links
• document the measured context reduction and recall tradeoff

Verification

• pnpm run verify
• 343 tests passed; 1 platform smoke test skipped locally
• deterministic build, Headless activation, package contents, publint, and attw passed