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>

AGENTS.md

@@ -61,6 +61,10 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   projects version-locked to the commit this repo pins. What deliberately did
   **not** move is the API surface and the config *schema*: routes, drivers,
   sessions and setups are what makes this project itself.
+- `SUBAGENTS.md` — a session's subagents as transcripts of their own
+  (`server/src/session/subagent.rs`, the subcards in `SessionListScreen.kt`
+  and the read-only form of `SessionScreen.kt`); `DECISIONS.md` holds the
+  choices made there that are still awaiting review.
 - `EXPLORER.md` — the file explorer's design (`server/src/files.rs` and
   `FilesScreen.kt` / `FileViewer.kt` / `FileEditor.kt`).
 - `TRANSCRIPT_CACHE.md` — the phone's copy of what it has been sent. Read it