fix(coding-agent): resolved agent:// slash form for nested subagent output

The agent:// path segment was always treated as a jq JSON-extraction key
against <parent>.md, so agent://Parent/Child loaded Parent.md and applied
.Child instead of resolving the nested child capsule Parent.Child.md. A
precise-planner reading its own scout child therefore got Not found.

The slash is now a hierarchy separator first: agent://Parent/Child resolves
Parent.Child.md (subagent children are allocated as dot-qualified ids). It
falls back to JSON extraction only when no nested output matches the path;
the ?q= query form is always extraction.

Fixes #5238

docs/tools/task.md

@@ -68,7 +68,7 @@ Settled response (`async.enabled=false`, no job manager, every item's agent `blo
 
 Artifacts and side channels:
 - Every subagent with an artifacts dir writes `<id>.md`; `agent://<id>` resolves to that file.
-- If the output file is JSON, `agent://<id>/<path>` and `agent://<id>?q=<query>` perform JSON extraction.
+- A subagent's own children are dot-qualified (`<id>.<child>`); `agent://<id>/<child>` reads that nested output. When the path names no nested output and the file is JSON, `agent://<id>/<path>` and `agent://<id>?q=<query>` perform JSON extraction.
 - Each subagent gets `<id>.jsonl` session history when the parent persists artifacts; `history://<id>` renders it as a concise transcript (works for live and parked agents).
 - Isolated patch mode writes `<id>.patch` before merge.