chore(release): pin K-brain beta.5 for LiveAgent beta.10

[xiaozhou26]

Summary

• Pin all desktop backend and computer-helper downloads to K-brain v0.107.6-beta.5 (be5bc2c), containing Refactor/base UI migration #23/ci: generate release notes with AI #24.
• Update release SHA-256 values for all four desktop targets; LA continues downloading release artifacts rather than compiling the backend.
• Release preparation for LiveAgent v2.0.0-beta.10, including fix(kbrain): 旧供应商导入被拒后不再每次启动重试，并在设置页给出可关闭的提示 #950/fix(kbrain): 旧会话迁移失败时给出可见清单，启动不再反复上传注定失败的会话 #951/fix(gateway): 未连接远程时发布队列快照不再报 outbound stream is offline #952.

Validation

• K-brain release CI passed; all 13 assets checked against GitHub digests and SHA256SUMS.
• Released macOS backend smoke: isolated startup/health, >5 MiB legacy import, stored session read, readable 64 MiB HTTP 413.
• Release prepare + packaging checks passed for macOS arm64/x64, Linux x64, Windows x64.
• Release/script tests: 44 passed.
• Full GUI regression suite rerunning against the new pinned backend before tagging.

Closes #955

Full GUI suite against the new pinned release: 3036 passed, 1 skipped, 0 failed.

[github-actions]

PR governance checks passed. Awaiting human review.

[xiaozhou26]

Full GUI regression suite passed against the new pinned K-brain release: 3036 passed, 1 skipped, 0 failed. Four-target release preparation and backend/helper checksum validation passed. Waiting for PR CI before merging and tagging beta.10.

[xiaozhou26]

First CI run hit an existing reconnect test timing race: TestV2KBrainRelayBrowserReconnect subscribed before later SSE events reached the gateway replay ring (latest_seq still equal to the first token). This PR changes only the backend lock file; no gateway code changed. The same test passed 30 consecutive local runs. Rerunning the failed Gateway job only; all other CI jobs passed. PR governance now passes with linked release tracking issue #955.