fix(subprocess): consumers with one mixed env map split DSH_* onto the managed channel

Codex review of the acp-only fix found lsp-local carries the identical
defect: its server config's unrestricted env merges into the connection's
ordinary spawn channel, so a configured DSH_* fact crashed the spawn with
the reserved-namespace rejection. The partition now lives on the seam as
splitEnvChannels() beside the scrub it complements; the ACP run and the
LSP connection both use it, and each proves child delivery end-to-end
(MOCK_ECHO_ENV / LSP_FAKE_ECHO_ENV fixture knobs). Seam + consumer README
rows updated (en+zh, re-recorded). bash-local is already two-channel;
mcp/pty/sdk bypass the seam and only share the scrub.

packages/subprocess/subprocess/tests/service.spec.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import { scrubbedParentEnv, SubprocessService } from '@deepseek-ai/dsh-subprocess'
+import { scrubbedParentEnv, splitEnvChannels, SubprocessService } from '@deepseek-ai/dsh-subprocess'
 import type { SubprocessDisposeGraces, SubprocessHandle, SubprocessOutputRead, SubprocessSpawnSpec } from '@deepseek-ai/dsh-subprocess'
 
 /**
@@ -72,4 +72,10 @@ describe('SubprocessService seam', () => {
       delete process.env.SCRUB_PROBE_PLAIN
     }
   })
+
+  it('splitEnvChannels partitions a mixed map onto the two spec channels', () => {
+    const { env, dshEnv } = splitEnvChannels({ DSH_FACT: 'managed', PLAIN: 'ordinary', DEEPSEEK_API_KEY: 'explicit' })
+    expect(env).toEqual({ PLAIN: 'ordinary', DEEPSEEK_API_KEY: 'explicit' })
+    expect(dshEnv).toEqual({ DSH_FACT: 'managed' })
+  })
 })