fix(docker): pi target sets OPENAB_AGENT_COMMAND=openab-agent but never ships that binary

[chaodu-agent]

Description

The pi agent target sets OPENAB_AGENT_COMMAND="openab-agent" but installs only pi-acp + @earendil-works/pi-coding-agent (npm) and copies only the openab binary — the openab-agent binary is never copied into the image. As a result the pi image can build and publish successfully but fails at runtime when OpenAB tries to launch its configured adapter (openab-agent: not found).

This is present across all three lanes:

• Dockerfile.pi (line ~49)
• Dockerfile.unified (pi target, ENV OPENAB_AGENT_COMMAND="openab-agent", only openab copied)
• Dockerfile.package (pi target, same)

Either the command should be the installed pi-acp (matching README, which lists pi as pi-acp), or the openab-agent binary must be copied into the pi image (as the native target does).

Discovered while adding the nightly build channel (#1519); pi is temporarily excluded from the nightly matrix to avoid publishing a runtime-broken canonical nightly-pi tag until this is fixed.

Steps to Reproduce

1. Build the pi image: docker build --target pi -f Dockerfile.package --build-context bins=./bin . (with a prebuilt openab in ./bin)
2. Run it with a config that starts the agent.
3. OpenAB attempts OPENAB_AGENT_COMMAND="openab-agent".

Expected Behavior

The pi image launches its ACP adapter successfully — either by setting OPENAB_AGENT_COMMAND="pi-acp" (the installed adapter) or by copying the openab-agent binary into the image if openab-agent is genuinely the intended driver for pi.