Pure resolution logic combining a workspace's AgentSpec with the
user-level default_agent into a Resolved value carrying Command, Args,
and Env. No I/O — PATH lookup stays in shell.ExecAgent and ctask agents
check, so Resolve is trivially testable and reusable.
BuiltinProfiles enumerates claude and opencode; "custom" is the escape
hatch and requires command. Keep BuiltinProfiles in sync with
workspace.knownAgentTypes and workspace.IsBuiltinAgentType (Task 1).
