feat(chat): view context — tell the assistant what you're looking at

An eye next to the paperclip shares what the user has open with their next
message: the page, the folder being browsed, the file open in the viewer and
any highlighted passage (line numbers where a source view exists), plus which
entity a detail page is about. The bag is client-authored {label, value} pairs
in English — the backend only clamps (chars, never bytes), neutralizes the
harness tag and renders one <system-extra> block per message, deduped
consecutively so it appears exactly when the view changed. On by default,
per-device toggle, hover/tap to preview, a chip on every sent message;
docs/view-context.md for users, an updated harness.md clause for the model.

crates/agent-loop/src/context.rs

@@ -15,7 +15,7 @@ use crate::activation::ActivationSource;
 use crate::ids::{ConversationId, FrameId};
 use crate::model::ModelInfo;
 use crate::projection::{
-    MediaSource, Projection, ProjectionHooks, ResultLimit, ToolResultDigest,
+    MediaSource, MessageExtras, Projection, ProjectionHooks, ResultLimit, ToolResultDigest,
 };
 use crate::store::HistoryStore;
 
@@ -157,6 +157,13 @@ impl LinearAssembler {
         self
     }
 
+    /// Text appended to each user/agent message (skipped media paths, the view
+    /// the message was sent from…). One hook, one block — see [`MessageExtras`].
+    pub fn with_extras(mut self, src: Arc<dyn MessageExtras>) -> Self {
+        self.hooks.extras = Some(src);
+        self
+    }
+
     /// How an over-long tool result is condensed.
     pub fn with_digest(mut self, digest: Arc<dyn ToolResultDigest>) -> Self {
         self.hooks.digest = Some(digest);