subagent: add -name flag, document roster convention

subagent_spawn now passes -name through to b, letting the parent assign
predictable session IDs. Updated coordination docs: parent passes full
peer roster in seeding prompts, names prefixed with parent session ID
for global uniqueness. Noted silent failure on duplicate names.

prompts/tools-subagent.md

@@ -14,6 +14,7 @@ execute_code: steps=[{tool: "subagent_spawn", args: ["-n", "N", "-agent", "A", "
 | Flag | Default | Description |
 |---|---|---|
 | `-n N` | 1 | Number of parallel subagents, all receiving the same prompt (homogeneous fan-out) |
+| `-name NAME` | auto | Session name; predictable IDs let you build a roster before agents start |
 | `-agent A` | server default | Agent config name |
 | `-backend B` | server default | Backend override |
 | `-model M` | server default | Model override |
@@ -53,10 +54,13 @@ general mandate.
 entry tracking the overall work. This is the one field the parent must always
 provide.
 
-**Peers** can be discovered by convention. All sessions live under `s/` and
-are named by session ID. Subagents spawned with `-n N` have predictable IDs.
-Rather than listing exact peers, describe how to discover them — e.g., "other
-subagents working in `$CWD`" — and let the child query `s/` if needed.
+**Peers** are known at spawn time. The parent creates the full roster and
+assigns `-name` to each subagent, so every peer ID is known before any agent
+starts. Pass the complete roster (name → task mapping) in each seeding prompt
+so agents can communicate directly without discovery. Names must be globally
+unique (they become directories under `s/`); prefix with the parent session ID
+to avoid collisions — e.g. `-name ${OLLIE_SESSION_ID}-impl`. Note: duplicate
+session names fail silently due to 9P client limitations.
 
 **Rendezvous** is implicit. Every session has a `prompt` file at
 `s/{sid}/prompt`. Writing to another session's prompt file always works:
@@ -74,21 +78,24 @@ fork-join; the prompt-based completion model is the intended direction.
 
 **Example coordination prompt**:
 ```
-You are the test-writer subagent. Spawned by session $PARENT_SESSION_ID.
-Shared task: beads issue #42 ("add auth middleware").
-A peer subagent is writing pkg/auth/middleware.go concurrently.
-When that file exists, write its tests to pkg/auth/middleware_test.go and run them.
-When done, write your result to s/$PARENT_SESSION_ID/prompt.
+You are the test-writer subagent (session: ${OLLIE_SESSION_ID}-test-writer).
+Parent: $OLLIE_SESSION_ID. Shared task: beads issue #42 ("add auth middleware").
+
+Roster:
+  ${OLLIE_SESSION_ID}-impl — writing pkg/auth/middleware.go
+  ${OLLIE_SESSION_ID}-test-writer (you) — writing pkg/auth/middleware_test.go
+
+When impl's file exists, write tests and run them.
+When done, write your result to s/$OLLIE_SESSION_ID/prompt.
 ```
 
 ## Heterogeneous fan-out
 
-To run N subagents with N *different* prompts, use a `{parallel: [...]}` stage. Each entry is an independent `subagent_spawn` call with its own prompt — use this when the subtasks differ:
+To run N subagents with N *different* prompts, use a `{parallel: [...]}` stage. Each entry is an independent `subagent_spawn` call with its own prompt and `-name` — use this when the subtasks differ and agents need to know about each other:
 
 ```
 execute_code: steps=[{parallel: [
-  {tool: "subagent_spawn", args: ["Summarize core/pkg/agent/loop.go"]},
-  {tool: "subagent_spawn", args: ["Summarize core/pkg/backend/anthropic.go"]},
-  {tool: "subagent_spawn", args: ["Summarize 9p/server.go"]}
+  {tool: "subagent_spawn", args: ["-name", "${OLLIE_SESSION_ID}-impl", "-agent", "coding", "-keep", "You are impl. Parent: $OLLIE_SESSION_ID. Roster: ${OLLIE_SESSION_ID}-impl (you) — write middleware, ${OLLIE_SESSION_ID}-test-writer — write tests. Task: beads #42. Write pkg/auth/middleware.go."]},
+  {tool: "subagent_spawn", args: ["-name", "${OLLIE_SESSION_ID}-test-writer", "-agent", "qa", "-keep", "You are test-writer. Parent: $OLLIE_SESSION_ID. Roster: ${OLLIE_SESSION_ID}-impl — write middleware, ${OLLIE_SESSION_ID}-test-writer (you) — write tests. Task: beads #42. When pkg/auth/middleware.go exists, write and run its tests."]}
 ]}], timeout=300
 ```

tools/subagent_spawn

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # description: Spawn N subagents with a shared prompt, wait for all to complete, return concatenated results (fork-join).
-# Args: [-n N] [-agent A] [-backend B] [-model M] [-cwd DIR] [-keep] <prompt...>
+# Args: [-n N] [-name NAME] [-agent A] [-backend B] [-model M] [-cwd DIR] [-keep] <prompt...>
 # Output: concatenated results from all subagents, in submission order. With -n > 1, results are separated by === {id} === headers.
 set -euo pipefail
 
@@ -13,6 +13,7 @@ n=1; args=()
 while [ $# -gt 0 ]; do
     case $1 in
         -n)       n=$2;                          shift ;;
+        -name)    args+=(-name    "$2");          shift ;;
         -agent)   args+=(-agent   "$2");          shift ;;
         -backend) args+=(-backend "$2");          shift ;;
         -model)   args+=(-model   "$2");          shift ;;