subagent: hand a subagent its parent's built-in tools, and no others

`build_config` spawned a subagent with `--dangerously-skip-permissions`
and no `--tools` at all, so it got claude's entire built-in set —
`SendMessage` and `ListAgents` (message peers, or the operator, as its
parent), `Task*` including `TaskStop`, which takes an *agent* id and so
reaches clean outside the run, `Cron*`, `RemoteTrigger` and
`EnterWorktree`/`ExitWorktree`. None of that is part of "do this bounded
task in this directory", and none of it is something the parent agent
itself can do: the harness has always passed `--tools`.

Pass the same one. The value comes from
`hive_sh4re::permissions::builtin_tools_arg()` — literally the function
the harness resolves its own session with — so the subagent's set is the
parent's set, `HIVE_TOOL_GROUPS` and all. That inheritance is the
requirement, not an implementation detail: a hardcoded subagent list
would hand `WebFetch`/`WebSearch` to the subagent of an agent without the
`web_tools` group, which is a privilege escalation, and would drift from
the parent's list the first time anyone added a tool to either.

`--tools` is the real gate: it holds under
`--dangerously-skip-permissions`, unlike `--allowedTools`, which only
auto-approves prompts. It does not filter MCP tools, so the
`goal_reached`/`need_help` signal surface is deliberately unnamed in it
and survives on `--strict-mcp-config` alone.

`build_config`'s doc comment claimed `strict_mcp_config` was *the* safety
property and that a subagent got "nothing implicit and nothing more".
That was false for built-ins, and is what hid this gap for as long as it
did; it now says which flag covers which half and that neither
substitutes for the other.

An empty `--tools` value parses as *unset* and grants more than omitting
the flag, so an empty resolution can only be a bug — `build_config`
asserts against it and a test pins the non-emptiness alongside the
subset-of-parent property.

Refs #4416

hive-subagent-mcp/Cargo.toml

@@ -13,6 +13,10 @@ axum.workspace = true
 clap.workspace = true
 hive-agent-sock.workspace = true
 hive-claude.workspace = true
+# `permissions::builtin_tools_arg` — the same `--tools` resolution the parent
+# harness spawns its own claude with, so a subagent's built-in surface is its
+# parent's rather than a second list that drifts. See `session::build_config`.
+hive-sh4re.workspace = true
 hive-sock-client.workspace = true
 hive-types.workspace = true
 libc.workspace = true