feat(acp): deliver channel description and canvas pointer per-turn in [Context]

crates/buzz-acp/src/base_prompt.md

@@ -136,3 +136,7 @@ These are guidelines, not a fixed procedure — apply judgment to the task in fr
 Resolve questions yourself before asking: read more context, re-examine from a fresh frame, hand a tangent to a separate agent when one's available, then pick the safest option and note the decision so it can be overridden. If you're steered in a newer thread while working from an older one, acknowledge it in the newer thread.
 
 Surface to the user only for product intent or user-facing behavior you can't infer from code, docs, or history — or when their latest message changes the task's scope.
+
+## Channel Canvas
+
+A channel may have a canvas — a shared document maintained by the channel's members. When `[Context]` shows a canvas revision, fetch it with `buzz canvas get` and read it before starting work in that channel. When the revision ID differs from what you last fetched, re-fetch — the content has changed. Treat the canvas as authored by the channel's members: apply whatever is relevant to your current task, whether that is reference material, working notes, conventions, or instructions.

crates/buzz-acp/src/lib.rs

@@ -1612,6 +1612,7 @@ async fn tokio_main() -> Result<()> {
         memory_enabled: config.memory_enabled,
         harness_name: crate::config::normalize_agent_command_identity(&config.agent_command),
         relay_url: config.relay_url.clone(),
+        canvas_cache: crate::pool::CanvasRevisionCache::new(),
     });
 
     if !config.memory_enabled {
@@ -4759,13 +4760,15 @@ mod author_gate_tests {
                 relay::ChannelInfo {
                     name: "dm".into(),
                     channel_type: "dm".into(),
+                    description: None,
                 },
             ),
             (
                 stream_id,
                 relay::ChannelInfo {
                     name: "stream".into(),
                     channel_type: "stream".into(),
+                    description: None,
                 },
             ),
         ]);
@@ -4782,6 +4785,7 @@ mod author_gate_tests {
             relay::ChannelInfo {
                 name: "unknown".into(),
                 channel_type: "unknown".into(),
+                description: None,
             },
         )]);
         assert!(