subagent_spawn: exponential backoff, structured output; fix stale docs

- Replace 5s/50ms poll with exponential backoff (~25s total)
- Output structured KV (session, path, backend, model) like cascade
- subagent_generate: remove stale result_path from header, document arg input
- Soften 'always use pipeline' to guidance matching actual behavior

prompts/tools-subagent.md

@@ -81,7 +81,7 @@ Remaining args: the task prompt (immediate task only — coordination lives in t
 - `[parent_plan_start]...[parent_plan_end]` — the parent's current plan file contents (if non-empty)
 
 **Constraints**:
-- **Always use the pipeline.** Never call `subagent_spawn` without `subagent_generate`.
+- **Prefer the pipeline.** Use `subagent_generate` → `subagent_spawn` when the subagent needs a custom identity. For simple delegation with no identity requirements, `subagent_spawn` alone with `-agent` or server defaults is fine.
 - **Identity carries coordination, prompt carries task.** Roster, shared task, and protocol go in the identity. The seed prompt is purely the task.
 - Subagents do not share memory. Pass context explicitly in the seed prompt.
 - Peers are known at spawn time. Assign `-name` to each and include the full roster in each identity.

tools/subagent_generate

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # description: Generate a JIT subagent config. No args prints the JSON template. With JSON on stdin, creates the config and prints the agent name.
-# Args: none
-# Stdin: JSON with structured fields (role, roster, shared_task, protocol, constraints, result_path) or legacy "prompt"
+# Args: [json] — JSON config as first arg, or piped via stdin
+# Stdin: JSON with structured fields (role, roster, shared_task, protocol, constraints) or legacy "prompt"
 # Output: template (no stdin) or agent name
 set -euo pipefail
 

tools/subagent_spawn

@@ -62,9 +62,13 @@ model=${model}"
 printf '%s\n' "$kv" > "$OLLIE/s/new" || die "failed to create session"
 
 B="$OLLIE/s/$name"
-SECONDS=0
-until [ -d "$B" ] || [ "$SECONDS" -ge 5 ]; do sleep 0.05; done
+wait=0.1
+for _ in 1 2 3 4 5 6 7 8; do
+    [ -d "$B" ] && break
+    sleep "$wait"
+    wait=$(awk "BEGIN{printf \"%.2f\", $wait * 2}")
+done
 [ -d "$B" ] || die "timeout waiting for session $name"
 
 printf '%s\n' "$prompt" > "$B/prompt" || die "failed to submit prompt"
-printf '%s\n' "$B"
+printf 'session=%s\npath=%s\nbackend=%s\nmodel=%s\n' "$name" "$B" "${backend:-default}" "${model:-default}"