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/TranscriptSource.kt

@@ -18,7 +18,7 @@ import java.util.concurrent.atomic.AtomicReference
  */
 class TranscriptSource(
     private val settings: ServerSettings,
-    private val sessionId: String,
+    private val address: TranscriptAddress,
     val cache: SessionCache,
 ) {
     private val stream = AtomicReference<EventStream?>(null)
@@ -65,7 +65,7 @@ class TranscriptSource(
         val tail = cache.tail() ?: return false
         // `before = seq + 1` is the newest event with seq <= the cursor, which is the event *at*
         // the cursor when the server still has one there.
-        val answer = fetchTranscript(settings, sessionId, before = tail.seq + 1, limit = 1)
+        val answer = fetchTranscript(settings, address, before = tail.seq + 1, limit = 1)
         val matches =
             answer.size == 1 &&
                 try {
@@ -83,7 +83,7 @@ class TranscriptSource(
      */
     suspend fun fetchOpening(): List<SeqEvent> {
         DebugStats.count("transcript page from server")
-        val page = fetchTranscript(settings, sessionId, limit = OPENING_WINDOW)
+        val page = fetchTranscript(settings, address, limit = OPENING_WINDOW)
         page.forEach { (line, entry) -> cache.append(line, entry.seq) }
         cache.flush()
         return page.map { it.second }
@@ -108,7 +108,7 @@ class TranscriptSource(
         val page =
             fetchTranscript(
                 settings,
-                sessionId,
+                address,
                 before = before,
                 limit = limit,
                 coalesce = coalesce,
@@ -131,7 +131,7 @@ class TranscriptSource(
      * well lose.
      */
     fun follow(after: Long, onOpen: () -> Unit, onReset: () -> Unit, onEvent: (SeqEvent) -> Unit) {
-        val opened = EventStream(settings, sessionId)
+        val opened = EventStream(settings, address)
         stream.getAndSet(opened)?.close()
         try {
             opened.run(after, onOpen, onReset) { raw, entry ->