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

Settings wrote the legacy secondary_model section through POST /config
and could not show what actually applied.

- The Agent tab replaces the model picker + pin switch with a Subagent
  Model Routing control: Inherit / Default / Pool / Force radios, a model
  picker for Default and Force, a checklist with a default for Pool, and
  an Effective routing card that shows the saved policy next to the
  policy that currently applies (with a line when the secondary-model
  feature is disabled).
- Reads and writes use GET/PUT/DELETE /config/subagent-model-policy with
  If-Match on the last read version; a 412 adopts the server's current
  state and surfaces the conflict instead of overwriting it.
- The HTTP client gains exchange() (extra request headers, response
  headers, allowed envelope codes). dist-web rebuilt.

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": "77096a8be7b91371d5d06d72292db9096968bd42e7b38e95f4adc9977e0cc6b1",
+  "sourceHash": "0fe4e070c0c5f5221f37b65b0abde459632df68d21f31624b441400ed46e1e47",
   "sourceFileCount": 405
 }