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/EventStream.kt

@@ -14,7 +14,7 @@ private const val RESET_EVENT = "reset"
  * mean. [close] from any thread ends it, and the caller owns reconnecting -- with the last seq it
  * saw as the new cursor.
  */
-class EventStream(settings: ServerSettings, private val sessionId: String) {
+class EventStream(settings: ServerSettings, private val address: TranscriptAddress) {
     private val stream = Sse(settings)
 
     fun close() = stream.close()
@@ -35,7 +35,7 @@ class EventStream(settings: ServerSettings, private val sessionId: String) {
         // one and the screen folds the other, and they have to be the same line.
         onEvent: (raw: String, event: SeqEvent) -> Unit,
     ) {
-        stream.run("/sessions/$sessionId/events?after=$after", onOpen) { name, data ->
+        stream.run("/${address.urlPath}/events?after=$after", onOpen) { name, data ->
             // A named frame carries no payload and a data frame has no name.
             if (name == RESET_EVENT) onReset()
             else if (data.isNotEmpty()) onEvent(data, parseSeqEvent(data))