fix(ci): prebuild workspaces in proxy publish verify job

[sriinnu]

Summary:

• add Build workspace packages step (npm run build:all) before package-local check/build/test in publish-proxy verify job
• update gate checkpoint and handover docs with second Gate 3 run result (run 22670212018) and current blocker state

Why:

• Gate 3 rerun after PR fix(ci): repair proxy publish selector workflow and refresh gate evidence #8 showed discover stage fixed, but verify jobs failed with workspace module resolution/typecheck errors before pack/publish dry-run
• prebuilding workspace packages ensures dist/type artifacts exist for cross-package type resolution during verify checks

Evidence:

• failing run analyzed: https://github.com/sriinnu/command-relay/actions/runs/22670212018
• local sanity: npm run build:all, npm --prefix packages/proxy-agent run check, npm --prefix packages/proxy-http-client run check (all pass)

Notes:

• governance blockers remain external: NPM_TOKEN, npm-publish environment, branch protection