ci: optimize Docker Smoke Test — build openab binary once, share across agent images

[thepagent]

Problem

The Docker Smoke Test workflow builds the same openab Rust binary 12 times in parallel (once per agent Dockerfile). Each build takes several minutes compiling identical code.

Proposal

1. Add a build-binary job that compiles openab once (or uses a cached artifact)
2. Each smoke-test job downloads the pre-built binary and only builds the agent-specific layer
3. This should cut total CI minutes by ~10x

Context

Noticed in PR #980 — the workflow triggers on any file change touching the PR paths and runs all 12 builds.

[github-actions]

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