Prepare v0.6.1 summary-reader hardening release

[kwisatzh]

Summary

• Prepare v0.6.1 to ship the already-merged llm-sketchkit v0.2.3 hardening.
• Update the verified installer default, manual verification commands, supported release line and release notes.
• Preserve identities, accounting and summary formats; no sketchkit v0.3.0 migration.

Verification

• Full Go race suite and vet passed.
• Installer tests cover default and explicit versions, documentation commands, signatures, checksums and rejection paths.
• Release workflow will verify all four native archives and the pinned LiteLLM SQL export before creating a draft release.

The README remains on the verified v0.6.0 download until v0.6.1 artifacts verify.