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.

docs/index.md

@@ -4,7 +4,7 @@ This folder is written for **you, the assistant**, not for the human directly. I
 
 Keep answers grounded in what's actually enabled and configured for this instance — check with the relevant tool (e.g. list installed/enabled plugins) rather than assuming everything described here is turned on. A feature documented here may not be enabled on this particular instance.
 
-This index will grow over time. Right now it covers the interface, files, agents, memory, projects, shared folders, background tasks, system agents, access grants, connectors, skills, the sandbox, voice input and plugins; more sections (security groups…) will be added later.
+This index will grow over time. Right now it covers the interface, view context, files, agents, memory, projects, shared folders, background tasks, system agents, access grants, connectors, skills, the sandbox, voice input and plugins; more sections (security groups…) will be added later.
 
 ## Features
 
@@ -24,6 +24,7 @@ This index will grow over time. Right now it covers the interface, files, agents
 | [skills.md](skills.md) | Skills: instruction folders the assistant loads on demand — where they live, how to read and run one, and the contract for writing, installing and downloading one |
 | [voice.md](voice.md) | Voice input: configuring a transcription model, and why the microphone button does nothing unless the page is served over HTTPS or localhost |
 | [interface.md](interface.md) | The desktop interface: collapsing the sidebar to an icon-only strip to make room for documents |
+| [view-context.md](view-context.md) | The eye in the chat: what "what you are looking at" means, what exactly gets shared with a message, how to turn it off, and where that text goes |
 
 ## Plugins