subagent_spawn: auto-cleanup JIT config; report resolved backend/model

- Remove JIT agent config from a/ after session creation (server has
  already loaded it), eliminating identity debris accumulation.
- Read actual backend/model from session cfg instead of echoing
  CLI-level defaults.
- Remove manual cleanup instruction from prompt (now automatic).

prompts/tools-subagent.md

@@ -86,7 +86,6 @@ Remaining args: the task prompt (immediate task only — coordination lives in t
 - 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.
 - Names must be globally unique under `s/`. Duplicate names fail silently.
-- Clean up when done: `rm $OLLIE/a/<name>.json`
 
 ## Returning results to the parent
 

tools/subagent_spawn

@@ -71,4 +71,11 @@ done
 [ -d "$B" ] || die "timeout waiting for session $name"
 
 printf '%s\n' "$prompt" > "$B/prompt" || die "failed to submit prompt"
-printf 'session=%s\npath=%s\nbackend=%s\nmodel=%s\n' "$name" "$B" "${backend:-default}" "${model:-default}"
+
+# Clean up JIT agent config now that the server has loaded it.
+[ -n "$agent" ] && rm -f "$OLLIE/a/${agent}.json" 2>/dev/null || true
+
+# Report resolved values from the live session.
+resolved_backend=$(grep -m1 '^backend=' "$B/cfg" 2>/dev/null | cut -d= -f2- || echo "${backend:-default}")
+resolved_model=$(grep -m1 '^model=' "$B/cfg" 2>/dev/null | cut -d= -f2- || echo "${model:-default}")
+printf 'session=%s\npath=%s\nbackend=%s\nmodel=%s\n' "$name" "$B" "$resolved_backend" "$resolved_model"