feat(13-01): Rust assistant spine — one curated tool, one backend, one RPC surface

D-01/D-02/D-06 tracer slice: a new crate::assistant module (CallerScope,
PermissionCategory, ToolExecCtx, chat()) runs a multi-turn tool-calling loop
(run_loop/execute_tool, MAX_TURNS=8) against a curated single-tool registry
(system_disk_status, hand-written JSON Schema — no schemars) via a Claude
Messages API backend. execute_tool is the single choke point: unknown tools
are refused not ignored, D-16 category grants are re-checked even though the
system prompt already omits ungranted tools, and every real tool dispatches
through the SAME handle_system_disk_status RPC handler every other
authenticated caller uses (assistant_dispatch_tool bridge in
api/rpc/assistant_chat.rs) — never an AI-only backdoor.

assistant.chat is registered in dispatcher.rs as a single guarded
`m if m.starts_with("assistant.")` arm reached only after the existing
session-cookie + CSRF + role.can_access() gate in api/rpc/mod.rs — asserted
directly by assistant_methods_require_session against the live
UNAUTHENTICATED_METHODS list (visibility only widened to pub(crate) for that
assertion; the list's contents are untouched, per the Phase-10 hard
constraint).

Key read from data_dir/secrets/claude-api-key — the same path
mesh/rpc/mesh/assistant.rs already probes — never a second key location.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/middleware.rs

@@ -2,7 +2,13 @@ use crate::session::SessionStore;
 use std::net::IpAddr;
 
 /// Methods that do not require a valid session cookie.
-pub(super) const UNAUTHENTICATED_METHODS: &[&str] = &[
+///
+/// `pub(crate)` (not just `pub(super)`) so `crate::assistant`'s test suite
+/// can assert directly against the live list that the assistant RPC prefix
+/// is never added to it (Phase-10 hard constraint) — see the re-export in
+/// `api/rpc/mod.rs`. Read-visibility only; the list's contents and every
+/// other visibility in this module are unchanged.
+pub(crate) const UNAUTHENTICATED_METHODS: &[&str] = &[
     "auth.login",
     "auth.login.totp",
     "auth.login.backup",