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>