Read the <!-- pk-commands BEGIN --> ... <!-- pk-commands END --> block
from the project's AGENTS.md. Extract the value for the build key.
If the value is a non-empty string, run it via Bash. Report the result: pass/fail status and a brief summary of the output (last 20 lines on failure, one-line confirmation on success).
If the value is an empty string "", tell the user: "No build command
configured for this project. Add it to the pk-commands block in
AGENTS.md."