Open transcript file links in explorer

EXPLORER.md

@@ -267,6 +267,18 @@ The speedometer went; the report is a "Copy render timings" row in
 already are. **Moving it is where the no-coordinate-taps rule got enforced**
 (Bryan, 2026-09-03) — see AGENTS.md's "Driving the UI".
 
+### 14. File links in a session open in the explorer
+
+A markdown destination that is an absolute path or a local `file:` URI opens that document in the
+session's explorer, on the session's setup. A trailing editor line and optional column are removed;
+the viewer opens the file but does not yet scroll to a line. Web links, relative links and `file:`
+URIs naming another host keep their ordinary external behaviour. The distinction is deliberately
+narrow: a relative link might be a web reference, and the phone must not silently reinterpret it as
+a path on another machine.
+
+The markdown link handler is provided around the session rather than taught about setups. That
+keeps the renderer reusable and makes the explorer's existing setup target the one navigation path.
+
 ## HTTP surface
 
 In `routes.rs`'s module doc with the rest. Bodies use `deny_unknown_fields`