hive-runtime, hive-agent: model/effort picker for ACP agents from configOptions

An ACP agent's model and effort pickers now list what its session offers
(its `model` and `thought_level` config options) instead of the claude
model list and EFFORT_LEVELS. A pick goes through the same Bus::set_model /
Bus::set_effort -> Config.model / Config.effort path as on claude; before
each prompt the ACP runtime sets it with `session/set_config_option`, model
first, and only when the session offers that value. Options are re-read
from the set response and from `config_option_update`, so the effort picker
disappears when the chosen model offers no effort levels, and is hidden
while a newly picked model waits for the next turn.

The session's options reach the web UI through a `Choices` handle from a
new `Runtime::choices`, registered on the bus the way `canceller` is.
/api/model and /api/effort accept only offered values on ACP. The claude
path is unchanged.

Refs #4391

CLAUDE.md

@@ -53,7 +53,7 @@ hand-maintained per-file tree drifts out of sync with the code.
   launch-config layer (tool-group/capability → `--allowedTools`,
   `--mcp-config` render).
 - **`hive-runtime/`** — the runtime an agent's turns run on: one
-  `Runtime` interface (`run`/`compact`/`archive`/`canceller`) with a `claude` backend
+  `Runtime` interface (`run`/`compact`/`archive`/`canceller`/`choices`) with a `claude` backend
   (a pass-through to `hive-claude`) and an `acp` backend (any Agent Client
   Protocol agent, spawned from the command/args/env nix hands it — no agent
   is named in Rust). The ACP backend translates its stream into claude's