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

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

packages/subagent/subagent-spawn/README.md

@@ -17,3 +17,22 @@ Spawn advertises `{ outputSchema: true, depthLimit: true, toolFilter: true, pers
 | Key | Meaning |
 |---|---|
 | `providerName` | Registry name on `ctx.subagents` (default `spawn`). |
+
+## Model Experience
+
+### Child-agent request
+
+**What the model sees**: The fresh child receives the standalone task content verbatim, inherits the parent model and workspace by default, and sees the global prompt with any configured child-scoped persona shadow. A tool filter removes global wire schemas, executable lookup, and Code Mode SDK bindings for that child but leaves independently registered guidance. It receives zero parent conversation messages; the filter is visibility/composition, not an authority grant inherited from the parent.
+
+**Token effect**: The child pays for a new independent context and history; no parent-history tokens are duplicated. Persona changes this child's repeated prompt cost, while filtering changes its schema or generated SDK cost.
+
+### Parent tool result, indirectly
+
+**What the model sees**: Through `dsh-tool-subagent`, the parent receives only the child's final output or stop-reason error.
+
+**Token effect**: Parent input grows by one data-dependent result retained until compaction.
+
+## Known Limitations and Deferred Work
+
+- **Runs expose no `sendMessage`/`resume`** — the optional runtime capabilities are absent on in-process runs.
+- **Fresh means no parent transcript** — the child inherits cwd, lineage, model, and explicitly configured persona/tool restrictions, but none of the parent's conversation; use the fork provider when completed-turn context is required.