fix: address human review — default providerName dsh-sdk; raise composition-e2e deadline

- subagent-sdk: the default registry name becomes `dsh-sdk` (the bare
  `sdk` read ambiguously in configs); READMEs, config catalog, fixture,
  and suites follow. The Loader fixture now omits providerName to exercise
  the shipped default end to end.
- loader-composition.e2e: two full harness runtimes boot in sequence, so
  the default 30s loader-smoke window times out under host load; raise the
  subprocess deadline to 120s with matching vitest headroom (the
  real-model.e2e precedent).

packages/subagent/subagent-sdk/src/index.ts

@@ -27,7 +27,7 @@ export const inject = ['subagents']
 
 /** Config: how to spawn and drive the child SDK runtime process. */
 export interface Config {
-  /** Provider name on `ctx.subagents` (default `sdk`). */
+  /** Provider name on `ctx.subagents` (default `dsh-sdk`). */
   providerName: string
   /** The executable to spawn for each run (the child runtime bin or packaged exe). */
   command: string
@@ -67,7 +67,7 @@ export interface Config {
 }
 
 export const Config: z<Config> = z.object({
-  providerName: z.string().default('sdk'),
+  providerName: z.string().default('dsh-sdk'),
   command: z.string().required(),
   args: z.array(z.string()).default([]),
   cwd: z.string(),