feat(web): Subagent Model Routing control backed by the policy endpoint (#240)

## Related Issue

No issue. Seventh step of the subagent routing work. Stacked on #239
(merge #234 → #239 first; until then this diff includes their commits).

## Problem

Settings wrote the legacy `secondary_model` section through `POST
/config` (merge semantics, no version check) and only showed the saved
value — never what actually applied when the secondary-model feature was
disabled or an environment override was in play.

## What changed

- Agent tab: the model picker + "Always use this model" switch are
replaced by **Subagent Model Routing** — Inherit / Default / Pool /
Force radios, the existing `SecondaryModelPicker` (model + thinking
effort) for Default and Force, a checklist with a chosen default for
Pool, and an **Effective routing** card that shows `Saved policy` next
to `Effective routing` plus `Secondary-model feature is disabled;
subagents inherit the main model` when the feature is off. Model choices
come from the model catalog (this screen selects models, not profiles).
- Reads/writes go through `GET/PUT/DELETE /config/subagent-model-policy`
with `If-Match` on the last read `resourceVersion`. A 412 adopts the
server's current state, surfaces the conflict as an operation failure,
and never overwrites the other writer.
- `DaemonHttpClient.exchange()` adds PUT/DELETE with extra request
headers, response headers, and allowed envelope codes.
`AppSubagentModelPolicy`/`AppSubagentModelPolicyState` types, wire
mappers, workspace-state actions (`loadSubagentModelPolicy`,
`saveSubagentModelPolicy`, `clearSubagentModelPolicy`) and client
exposure.
- The `force: false` compatibility test from #234 is removed with the
legacy path (the web and gateway ship in one bundle, so there is no
version skew to defend).
- `dist-web` rebuilt.

Tests: section shows only once the policy state is loaded and sits after
the default model; one payload per mode (inherit → clear, default/force
→ picked model + effort, pool → models map with default); saved vs
effective card and the feature-disabled line; workspace state writes
with the last read version and adopts the server state on a 412.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [ ] I have linked a related issue (external PRs: the issue must have a
maintainer's `/approve`).
- [x] I have added tests that prove my feature works.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added subagent model routing controls with Inherit, Default, Pool, and
Force modes.
* Settings now show both the saved routing policy and currently applied
routing.
* Added options to fold completed turns and consecutive tool activity
runs.
* Tasks can be detached from the foreground and continue running in the
background.
  * Added clearer experimental-feature status indicators.

* **Bug Fixes**
* Improved recovery when saved routing settings conflict with a newer
version.
* Improved handling of task and routing updates received during active
sessions.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/settings-subagent-routing.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+Settings gains a Subagent Model Routing control with Inherit, Default, Pool, and Force modes and shows the saved policy next to the routing that currently applies.

apps/pythinker-code/dist-web/.web-bundle-manifest.json

@@ -1,4 +1,4 @@
 {
-  "sourceHash": "f7f3ed6157809fd616d25b246300a55af33b2e2893911910ae1a2742c790b499",
+  "sourceHash": "c294d9482527fbd47119352fcaac10f9870f9ef62d7228be22642ea8bd1a7804",
   "sourceFileCount": 407
 }