permissions: give the built-in tool list one home, next to ToolGroup

The `--tools` list a harness session gets is not a constant: the base set
plus whatever the agent's `HIVE_TOOL_GROUPS` add (today, `web_tools` →
`WebFetch`/`WebSearch`). That resolution lived in `hive-agent`'s
`mcp_config`, which is fine while the harness is the only thing that
spawns a `claude` — and it is not: `hive-subagent-mcp` spawns one too.

`hive-agent` is binary-only (no `src/lib.rs`, no lib target), so nothing
can depend on it to reach `builtin_tools_arg`. The alternative to a
shared home is a second list in the subagent daemon, which diverges on
the first tool anyone adds to either — and diverging upward is a
subagent holding a built-in its parent does not have.

So move the base list, the `HIVE_TOOL_GROUPS` parse and the resolution
into `hive_sh4re::permissions`, beside `ToolGroup` — whose
`builtin_tools()` was already half of the answer. `hive-agent`
re-exports them, so `mcp_config::builtin_tools_arg()` still reads the
same at the call site, and `allowed_tools_arg` now derives its built-in
half from the same function rather than repeating the merge loop.

Behaviour is unchanged. The parse is `strum::EnumString` rather than a
`serde_json::from_value` round-trip through a `Value::String`: same
`snake_case` names (a test pins the two derives against each other),
without `hive-sh4re` needing `serde_json` outside its dev-dependencies.
It is now a pure function of its input, so the fallbacks are testable
without mutating the environment — which under edition 2024 is `unsafe`
and racy across a test binary's threads.

Refs #4416

docs/turn-loop/mcp.md

@@ -203,7 +203,12 @@ than deriving from SSE events.
 runs the body, logs the result. Pre-/post-log only — the inbox
 status hint lives in the wake prompt + UI header, not here.
 
-## Tool allowlist (`mcp_config::ALLOWED_BUILTIN_TOOLS`)
+## Tool allowlist (`hive_sh4re::permissions::ALLOWED_BUILTIN_TOOLS`)
+
+The built-in list and the `--tools` value it resolves to live in
+`hive-sh4re` alongside `ToolGroup`, not in the harness, because the
+subagent daemon spawns its own `claude` and must resolve the same set —
+see [`docs/tools/subagent.md`](../tools/subagent.md).
 
 - Allowed built-ins: `Edit`, `Glob`, `Grep`, `Read`, `Skill`, `Write`.
   `Skill` is what makes an installed plugin's `SKILL.md` invokable —