require a2a pack write prefix support

agent_builder/skills/deepagent-agent-design/SKILL.md

@@ -17,7 +17,8 @@ Use this shape for non-trivial generated agents:
    DeepAgent with `create_deep_agent`.
 3. Pass `backend=ctx.workspace_backend()` so DeepAgents file tools write to the
    caller's durable workspace instead of LangGraph state.
-4. Pass `skills=[RUNTIME_SKILLS_ROOT]` when the generated project includes
+4. Seed project skills into the current grant's write prefix and pass
+   `skills=skill_sources or None` when the generated project includes
    `skills/<skill-name>/SKILL.md`.
 5. For custom subagents, include a `skills` field on each subagent definition.
    The general-purpose subagent inherits main skills, but custom subagents do
@@ -47,32 +48,46 @@ before `create_deep_agent`:
 
 ```python
 from pathlib import Path
+from typing import Any
 
-RUNTIME_SKILLS_ROOT = "/outputs/{{ agent_name }}/.deepagents/skills/"
+RUNTIME_SKILLS_DIR = "{{ agent_name }}/.deepagents/skills/"
 
 
-def _seed_runtime_skills(backend: Any) -> None:
+def _runtime_skills_root(ctx: Any) -> str:
+    workspace = getattr(ctx, "_workspace", None)
+    prefixes = tuple(getattr(workspace, "write_prefixes", ()) or ())
+    if not prefixes:
+        outputs_prefix = getattr(workspace, "outputs_prefix", None)
+        prefixes = (outputs_prefix or "outputs/",)
+    prefix = str(prefixes[0]).strip("/")
+    return f"/{prefix}/{RUNTIME_SKILLS_DIR}" if prefix else f"/{RUNTIME_SKILLS_DIR}"
+
+
+def _seed_runtime_skills(backend: Any, ctx: Any) -> list[str]:
     root = Path(__file__).parent / "skills"
     if not root.exists():
-        return
+        return []
+    runtime_skills_root = _runtime_skills_root(ctx)
     uploads: list[tuple[str, bytes]] = []
     for path in root.rglob("*"):
         if path.is_file():
             rel = path.relative_to(root).as_posix()
-            uploads.append((RUNTIME_SKILLS_ROOT + rel, path.read_bytes()))
+            uploads.append((runtime_skills_root + rel, path.read_bytes()))
     if uploads:
         backend.upload_files(uploads)
+        return [runtime_skills_root]
+    return []
 ```
 
 Then:
 
 ```python
 backend = ctx.workspace_backend()
-_seed_runtime_skills(backend)
+skill_sources = _seed_runtime_skills(backend, ctx)
 return create_deep_agent(
     model=model,
     backend=backend,
-    skills=[RUNTIME_SKILLS_ROOT],
+    skills=skill_sources or None,
     tools=[small_deterministic_tool],
     subagents=[...],
     system_prompt=SYSTEM_PROMPT,