A subagent inherited the parent's built-in tool list, which correctly has
no `Bash` -- the agent reaches a shell through the `bash` MCP server, not
the built-in. Subagents get no such server, so the intersection was empty
and they could not run a command at all: no commits, no pushes, no gates.
Add `subagent_builtin_tools_for`/`_arg`, which reuse the shared resolver
and append `Bash` only when `Execution` -- the group that gates the `bash`
MCP server -- is present. Only the subagent spawn path calls them, so the
harness's own `--tools`/`--allowedTools` are unchanged.
The capability transfers; the mechanism does not.
Refs #4422
5ce0a357b4