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/assistant/backends/mod.rs

@@ -0,0 +1,35 @@
+//! The `Backend` trait — the wire-format-agnostic seam every model backend
+//! (Ollama, Claude, Routstr) implements once. The loop and every tool are
+//! written against this trait only; wire-format differences live entirely
+//! inside each adapter.
+
+use std::path::Path;
+
+use anyhow::Result;
+use async_trait::async_trait;
+
+use super::tools::{ChatMessage, ToolCall, ToolDef};
+
+pub mod claude;
+#[cfg(test)]
+pub mod scripted;
+
+pub enum BackendTurn {
+    Text(String),
+    ToolCalls(Vec<ToolCall>),
+}
+
+#[async_trait]
+pub trait Backend: Send + Sync {
+    async fn send(&self, system: &str, tools: &[ToolDef], history: &[ChatMessage]) -> Result<BackendTurn>;
+}
+
+/// D-04's backend chain: local Ollama first (node data never leaves the
+/// node when a local model is available), then Claude, then Routstr. Only
+/// the Claude leg is implemented in this tracer — `backends/ollama.rs`
+/// (13-10) and `backends/routstr.rs` (13-13) slot in ahead of and behind it
+/// without changing the `Backend` trait; that is the architectural
+/// commitment this tracer proves.
+pub fn select_backend(data_dir: &Path) -> Box<dyn Backend> {
+    Box::new(claude::ClaudeBackend::new(data_dir.to_path_buf()))
+}