fix(custom-model): isolate Claude config dir and inject real context length

Addresses two live-validation findings on the Run-menu custom-model picker:

1. Both claude.ai and ANTHROPIC_API_KEY set warning. Claude Code still
   coexists an OAuth login with an injected ANTHROPIC_API_KEY in the same
   config directory and warns about it (confirmed cosmetic - the API key
   wins for actual requests, verified via a real session's own API Usage
   Billing line). A custom-model claude session now gets an isolated
   CLAUDE_CONFIG_DIR (registry-declared via a new configDirVar field, empty,
   no files written into it) so there is nothing to conflict with. projects
   is symlinked (junction on Windows) back into the real config dir so the
   response viewer, subagent windows and Read My Mind keep working for that
   session, best-effort.

2. Context-window overflow. Claude Code assumes a large default context
   window for a model id it doesn't recognise and never compacts, so a
   custom endpoint's real, much smaller context (verified live: a 400
   exceeding a 16384-token llama-swap model with a stock ~33.7K-token system
   prompt) silently overflows. Discovery now also learns each model's real
   context length from llama.cpp/llama-swap's GET /props?model=<id> (n_ctx),
   but ONLY for a model llama-swap's own /v1/models response already marks
   status.value === 'loaded' - never an unloaded one, since llama-swap
   treats ?model= as a routing hint and probing an unloaded model risks
   triggering an actual, slow, GPU-swapping load as a side effect of
   read-only discovery. A server with no status field at all gets no
   enrichment rather than a guess; a model not probed this round keeps its
   previously-learned value until it disappears from the list entirely.
   Stored per model (CustomModelHost.modelContextLengths) and applied via a
   new contextLengthVar registry field, set to
   CLAUDE_CODE_MAX_CONTEXT_TOKENS for claude.

Both new fields live on the existing env-kind customModelInjection
capability shape, declared only on claude's registry entry - every other
CLI's injection is unaffected (pinned by test).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

src/custom-model-injection.ts

@@ -44,6 +44,14 @@ export interface EnvInjection {
   envOverrides: Record<string, string>;
   /** See {@link ConfigDirInjection.launchModel}. */
   launchModel?: string;
+  /**
+   * Name of the env var the caller should point at an isolated, credential-free config
+   * directory for this session (claude's `CLAUDE_CONFIG_DIR`), from the registry entry's
+   * `customModelInjection.configDirVar`. The actual directory value isn't computed here —
+   * this module is pure and has no sessionId to derive one from — the IO wrapper
+   * (`custom-model-injection-apply.ts`) creates it and adds it to `envOverrides`.
+   */
+  configDirVar?: string;
 }
 
 export interface ConfigDirInjection {
@@ -90,7 +98,9 @@ function quoted(value: string): string {
 export function buildCustomModelInjection(
   entry: Pick<CliEntry, 'capabilities'>,
   endpoint: CustomModelEndpoint,
-  modelId: string
+  modelId: string,
+  /** Discovered context-window size for `modelId`, if known — see `contextLengthVar`. */
+  contextLength?: number
 ): CustomModelInjectionResult {
   const cap = entry.capabilities.customModelInjection;
   const apiKey = endpoint.apiKey?.trim() || DEFAULT_API_KEY;
@@ -102,7 +112,11 @@ export function buildCustomModelInjection(
         [cap.apiKeyVar]: apiKey,
       };
       for (const modelVar of cap.modelVars) envOverrides[modelVar] = modelId;
-      return withLaunchModel({ kind: 'env', envOverrides }, cap.launchModel, modelId);
+      if (cap.contextLengthVar && contextLength !== undefined && Number.isFinite(contextLength)) {
+        envOverrides[cap.contextLengthVar] = String(Math.trunc(contextLength));
+      }
+      const result = withLaunchModel({ kind: 'env', envOverrides }, cap.launchModel, modelId);
+      return cap.configDirVar ? { ...result, configDirVar: cap.configDirVar } : result;
     }
 
     case 'configContentEnv': {