Merge branch 'codex/simp-agent-entry-state' into codex/simp-unify-agent-session-id

# Conflicts:
#	packages/ui/stdio-agent/README.md

packages/subagent/tool-subagent/README.md

@@ -26,3 +26,41 @@ A non-`completed` stop reason becomes an `isError` tool result; partial child ou
 | `maxDepth` | Absolute delegation-depth cap; requires provider `depthLimit` capability. |
 
 `toolFilter` changes the child's visible global tool layer; it is not a parent-derived authority ceiling. See the [agent-scope security non-goal](../../../docs/rfc/implemented/architecture/2026-07-08-agent-scope-contexts.md#security-and-authority-are-explicit-non-goals).
+
+## Model Experience
+
+### Standalone-provider schema
+
+**What the model sees**: While a fresh-context provider exists, the configured tool uses the generated default [`subagent` schema](../../../docs/tool-catalog.md#deepseek-aidsh-tool-subagent); the catalog also records how `toolName` changes the visible name.
+
+**Token effect**: Fixed schema cost per parent request while mounted. Removing the provider removes the whole schema.
+
+### Inherited-context-provider schema
+
+**What the model sees**: Relative to the generated default [`subagent` schema](../../../docs/tool-catalog.md#deepseek-aidsh-tool-subagent), a provider that seeds completed turns replaces only the tool and `prompt` parameter descriptions with the text below; the shape and `description` parameter stay unchanged.
+
+**Token effect**: Fixed schema cost per parent request while mounted. Exposing multiple providers adds one independently named schema per load.
+
+#### Inherited-context-provider tool description
+
+```markdown
+Delegate a task to a subagent that INHERITS this conversation: a child agent seeded with all completed turns so far (it does not see the current in-flight turn), returning only its final result. Use this when the subtask builds on this conversation's context — a follow-up analysis, a review, a continuation — without consuming this conversation's context for the work itself. You receive only its final answer, not its intermediate steps.
+```
+
+#### Inherited-context-provider prompt description
+
+```markdown
+The task for the subagent. It already sees this conversation's completed turns, so build on them freely and state only what is new.
+```
+
+### Tool-call history and result
+
+**What the model sees**: The task description and full prompt remain in the parent assistant tool call. Success contains only the child's data-dependent final text. Other stop reasons become exactly `Error: subagent run was cancelled`, `Error: subagent run failed`, `Error: subagent run hit its token limit before finishing`, `Error: subagent declined the task`, or `Error: subagent run ended abnormally (<reason>)`; a call without an owning agent becomes `Error: subagent tool requires a calling agent (exec.agent was undefined)`. Intermediate child steps never enter the parent.
+
+**Token effect**: Prompt and final output are data-dependent retained tokens. All child working context is paid in the child and omitted from the parent.
+
+## Known Limitations and Deferred Work
+
+- **Delegation blocks the parent turn** — synchronous collect only; background start and poll collection are deferred to the long-running-runtime redesign.
+- **Duplicate `toolName` across waiting loads is detected late** (`TODO(subagent-dup-toolname)`) — two loads waiting on providers collide only when a provider arrives, and the throw rolls back the provider's fiber rather than the misconfigured tool's; config-time detection needs a cross-fiber registry of intended names.
+- **Child policy is fixed per tool registration** — `model`, persona, tool filter, and depth cap come from this plugin load's config, not model-call arguments; exposing another policy requires another distinctly named tool.