feat(coding-agent): Virtual models (#10035)

This adds experimental support for virtual models. A virtual model is a catalog entry that an extension registers with pi.registerVirtualModel(). It does not talk to a provider itself; for every request it picks a physical model and thinking level. The motivation is to make routing policies pluggable: plan on a strong model and implement on a cheap one, pick a model with a classifier, or move to a larger window when the context grows. Today this needs model switching by hand or hacks in prepareRequest.

The selection stays virtual: model_change records it, resume restores it, and each response records the physical model and thinking level that produced it. Router state can be stored on the session branch, so it follows /tree and forks and survives compaction. Context limits, compaction and image sizing use the physical model a request was routed to. examples/extensions/jev-router.ts shows a full router that uses the Jev classifier to pick Sol or Terra for planning and hands off to Luna after the first edit. docs/virtual-models.md describes the API.

packages/coding-agent/docs/extensions.md

@@ -80,6 +80,7 @@ Automatic retries, recovery, compaction, or queued work can continue afterward.
 | Persist non-context session data | `pi.appendEntry()` |
 | Change active tools, model, or thinking level | Session control methods on `pi` |
 | Add a model provider | `pi.registerProvider()` |
+| Route each request to a model | [`pi.registerVirtualModel()`](virtual-models.md) |
 | Add terminal rendering | Renderer registration and `ctx.ui` |
 | Communicate with another extension | `pi.events` |