propose(agentic): read agent_workspace_root from request body and persist to state

Completes the UI<->soul contract for #23 (scope file/command tools to an agent
workspace root). #23 made the tools read state_get("agent_workspace_root"), but
nothing set that key from the desktop UI, so the agent panel's Workspace Folder
was cosmetic and tools ran unscoped (default-allow). This reads the root the UI
now sends on each agentic request and state_sets it before tool dispatch, so
agent_workspace_root() picks it up for the turn.

Minimal + pattern-matching (same json_get/state_set shape used throughout chat.el).
Empty body field => unscoped (backward-compatible) and preserves the env fallback.

FOR WILL'S REVIEW — do not merge without sign-off:
- Ownership model: set state from body each turn (so clearing the folder un-scopes)
  vs. only-when-nonempty. Flagged inline.
- Pairs with neuron-ui PR #32 (ChatRequest.agentWorkspaceRoot).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

chat.el

@@ -631,6 +631,17 @@ fn handle_chat_agentic(body: String) -> String {
         return "{\"error\":\"message required\",\"reply\":\"\"}"
     }
 
+    // Workspace scope (#23): the desktop UI sends the user-chosen Agent Workspace root
+    // on every agentic request. Persist it to state so agent_workspace_root() — and the
+    // path/command tool guards that read it — confine this turn's file/command tools to
+    // that subtree. The UI is the source of truth per request: empty means unscoped (the
+    // backward-compatible default), and it also lets agent_workspace_root() fall through
+    // to the NEURON_AGENT_ROOT env when no root is sent. FLAGGED FOR REVIEW: setting
+    // state from the body each turn (vs. only-when-nonempty) so clearing the folder in
+    // the UI un-scopes — confirm this is the intended ownership model.
+    let ws_root: String = json_get(body, "agent_workspace_root")
+    state_set("agent_workspace_root", ws_root)
+
     let req_model: String = json_get(body, "model")
     let model: String = if str_eq(req_model, "") { chat_default_model() } else { req_model }