feat(sessions): a dsh session shows its route config's model while its screen names none

displayModel gains a `config` source, ranked below any report from the running
CLI and above the launch model: custom endpoint, then statusline or screen,
then config, then launch, then nothing. The screen still wins whenever it
names a model, since that is what the running TUI uses.

- Registry data: capabilities.modelDetect gains `configResolver`, a NAMED
  reader (src/model-config-resolvers.ts), like a launcher profile; dsh names
  'deepseek-route' (the reader from the previous commit). `screenLine` becomes
  optional; the schema refuses a modelDetect naming nothing, an unknown
  reader, or screenLines without a screenLine.
- Session: the reader runs from _withPaneLifecycle's finally, so at every pane
  start, attach and relaunch, with the session's own launch config
  (legacyConfigForMode) and env (its clamped overrides, then the server's), so
  a per-session DSH_HOME is the home read. Async; a read that lands after a
  newer one or after the session stopped is dropped; a remote or docker
  session reads nothing locally. A change emits displayModelChanged
  (broadcast and persist). Not restored after a restart: the next attach
  reads it again, and a restored screen value outranks it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/config/cli-registry/schema.ts

@@ -15,7 +15,7 @@
 import { z } from 'zod';
 import { compileVersionRegex, countCaptureGroups, TOKEN_PATTERNS } from './patterns.js';
 import { isKnownLauncherProfile, isKnownSetenvProfile } from './profiles.js';
-import type { McpConfigFormat } from './types.js';
+import type { McpConfigFormat, ModelConfigResolverName } from './types.js';
 
 /** A bare CLI id: lowercase, starts with a letter, at most 24 chars. Also used as a CSS/URL token. */
 const cliId = z
@@ -382,12 +382,24 @@ const capabilitiesSchema = z
           .refine(
             (src) => compileVersionRegex(src) !== null && countCaptureGroups(src) === 1,
             'screenLine must be a regex compileVersionRegex() accepts (at most 200 characters, no nested quantifiers) with exactly one capture group'
-          ),
+          )
+          .optional(),
         // Bounded hard, like watchingLines: every row it adds is one more row the agent
         // itself may be able to write.
         screenLines: z.number().int().min(1).max(4).optional(),
+        // A NAMED reader (src/model-config-resolvers.ts), never code in config.
+        configResolver: z.enum(['deepseek-route'] as const satisfies readonly ModelConfigResolverName[]).optional(),
       })
       .strict()
+      // Typos rather than configurations, refused at LOAD time like watchingLines.
+      .refine(
+        (v) => v.screenLine !== undefined || v.configResolver !== undefined,
+        'modelDetect declares nothing to read'
+      )
+      .refine(
+        (v) => v.screenLines === undefined || v.screenLine !== undefined,
+        'screenLines has nothing to bound without a screenLine'
+      )
       .optional(),
     privilegedParams: z
       .array(