docs: scrub MCPLOCAL_MCPD_TOKEN — pod has no persistent mcpd identity

The earlier plan recommended an MCPLOCAL_MCPD_TOKEN env var so the pod
would have a ServiceAccount session into mcpd. It's unnecessary: the
pod forwards every inbound client bearer (mcpctl_pat_...) verbatim to
mcpd for all downstream calls — both introspect and project discovery.
mcpd's auth middleware dispatches on the prefix and resolves the
McpToken principal directly. No pod secret, no rotation story.

Updates:
- serve.ts header: explicit "identity model" section calling this out
  so future readers don't restore the env var thinking it's missing.
- docs/mcptoken-implementation.md: drop the "mount MCPLOCAL_MCPD_TOKEN"
  Pulumi guidance and the "dedicated ServiceAccount" follow-up item;
  state the correct image URL (internal 10.0.0.194 registry) and the
  gated-vs-ungated rule for LLM config mounts.

No runtime code changes — serve.ts never actually required the token;
this just fixes the documentation and the header comment.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/mcplocal/src/serve.ts

@@ -9,6 +9,14 @@
  *   - Requires MCPLOCAL_MCPD_URL to point at mcpd inside the cluster.
  *   - Registers a token-auth preHandler on `/projects/*` and `/mcp`.
  *   - FileCache directory honours MCPLOCAL_CACHE_DIR (wired via project-mcp-endpoint).
+ *
+ * Identity model: **the pod has no persistent identity to mcpd.** Every
+ * inbound request's `Authorization: Bearer mcpctl_pat_…` is forwarded
+ * verbatim for all downstream mcpd calls (introspect + project
+ * discovery). mcpd's auth middleware dispatches on the `mcpctl_pat_`
+ * prefix and resolves the McpToken principal. As a result there is
+ * deliberately no MCPLOCAL_MCPD_TOKEN env var — adding one would only
+ * create a rotation problem for a state we don't need.
  */
 import { McpRouter } from './router.js';
 import { createHttpServer } from './http/server.js';