feat(desktop): import Shepherd execution evidence

[gagan114662]

Summary

• add an optional Shepherd flat-JSON trace importer to the desktop backend
• normalize external effects into a Buzz-owned evidence envelope
• redact raw prompts, tool results, and file content while retaining hashes and useful boundary metadata
• enforce import size, effect-count, sequence, and effect-type consistency checks
• expose the importer through the Tauri command surface without requiring Python or Shepherd at runtime

Why

Shepherd is early alpha and its public replay API is not shipped. This change takes the smallest stable integration seam available today: structured evidence import. Buzz retains ownership of identity, approvals, explanations, and the universal evidence format.

Verification

• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml -- --check
• full desktop Rust library suite at commit 1a461baa9bcbd5073d307b6f3ce7a90447e4d298: 2,234 passed, 0 failed, 13 ignored

Privacy and compatibility

• raw Shepherd event payloads are not returned
• events carry a SHA-256 payload digest for correlation
• the adapter is optional and adds no Shepherd or Python dependency
• coverage is explicitly labeled boundary-effects-only

Originating Buzz channel: 2db0f46d-71e4-46c2-9798-3b248dac5fff