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/mod.rs

@@ -1,5 +1,6 @@
 mod analytics;
 mod ark;
+mod assistant_chat;
 mod auth;
 mod backup_rpc;
 mod bitcoin;
@@ -59,9 +60,15 @@ use std::sync::Arc;
 use tracing::{debug, error};
 
 pub use middleware::PeerAddr;
+// Re-exported `pub(crate)` (not just imported) so `crate::assistant`'s test
+// suite can assert directly against the live list that `assistant.*` is
+// never added to it — the Phase-10 hard constraint this crate must hold.
+// The list's *contents* are unchanged; only its read-visibility widens from
+// "this module" to "this crate".
+pub(crate) use middleware::UNAUTHENTICATED_METHODS;
 use middleware::{
     derive_csrf_token, extract_client_ip, extract_cookie, sanitize_error_message,
-    CACHEABLE_METHODS, UNAUTHENTICATED_METHODS,
+    CACHEABLE_METHODS,
 };
 use response::{cookie_header, json_response, ResponseCache, RpcError, RpcRequest, RpcResponse};