fix: preserve agent field during command expansion

[dgageot]

When invoking a slash command that targets a sub-agent via the agent: field, the routing was incorrectly going to the root agent instead of the named sub-agent. The root cause was in pkg/js/expand.go's ExpandCommands function, which rebuilt the types.Command struct field-by-field at config-load time and silently dropped the Agent field. When LookupCommand later tried to route the command, it saw an empty Agent and never switched agents.

This change fixes the bug by preserving the Agent field during command expansion. A follow-up refactor makes ExpandCommands copy the struct and overwrite only the expanded text fields (Description, Instruction), ensuring that future fields are preserved automatically and preventing this class of bug from recurring.

The fix includes unit tests in pkg/js/expand_agent_test.go and an end-to-end test in e2e/exec_command_agent_test.go with a recorded cassette and config fixture, verifying that the instruction reaches the specialist sub-agent directly. All validation checks pass: build, go vet, race tests, and the full test suites for js, cli, runtime, config, and e2e.

[docker-agent]

Assessment: 🟢 APPROVE

The fix in pkg/js/expand.go is correct and well-tested. Copying the types.Command struct and overwriting only the Description and Instruction text fields reliably preserves the Agent field and prevents this class of bug from recurring with future fields.

The new unit test (TestExpandCommandsPreservesAgent) directly covers the regression, and the e2e test (TestExec_CommandTargetsAgent) validates the full routing path end-to-end. No bugs found in the added code.