Show a session's subagents as subcards, each with a read-only transcript

A subagent is a second transcript owned by a session, in the same event
model, with no process and no controls. The claude translator routes lines
carrying parent_tool_use_id to a per-subagent translator and transcript
under <session>/subagents/<tool_use_id>; three routes expose the list, a
transcript page and the SSE stream. Echo grows /subagent [n] as the rig.

On the phone a card with subagents ends in a chevron expander, collapsed by
default, opening to outlined subcards styled like dev-updater's components;
a subcard opens SessionScreen in read-only form, addressed through
TranscriptAddress so paging, cache and stream are shared.

Design in SUBAGENTS.md; choices awaiting review in DECISIONS.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptAddress.kt

@@ -0,0 +1,27 @@
+package com.example.aiapp
+
+/**
+ * Where one transcript lives: a session's own, or one of its subagents'.
+ *
+ * The single mechanism [fetchTranscript], [EventStream], [TranscriptSource] and
+ * [TranscriptCache.session] all take, rather than each growing its own branch between a session and
+ * a subagent -- see SUBAGENTS.md's "Phone" and "Wire shape". A caller that has only a session id
+ * builds one with the one-argument constructor; a subagent's screen supplies both ids.
+ */
+data class TranscriptAddress(val sessionId: String, val subagentId: String? = null) {
+    /** The URL segment naming this transcript, before `/transcript` or `/events`. */
+    val urlPath: String
+        get() =
+            if (subagentId == null) "sessions/$sessionId"
+            else "sessions/$sessionId/subagents/$subagentId"
+
+    /**
+     * Where this transcript's cache lives on the phone, relative to the cache root.
+     *
+     * A subagent's nests under its session's directory rather than sitting beside it, so deleting a
+     * session's cache directory takes its subagents' with it -- the same one-way door the server's
+     * own storage describes.
+     */
+    val cachePath: String
+        get() = if (subagentId == null) sessionId else "$sessionId/subagents/$subagentId"
+}