subprocess: one explicit env channel on the spawn spec

Drop SubprocessSpawnSpec.dshEnv and splitEnvChannels(); childEnv() is now
scrubbed-base + explicit entries with no namespace validation. The invariant
dropped is the reserved-namespace check on explicit entries (DSH_* rejected
from env, non-DSH_* rejected from dshEnv). Explicit-entry trust already
covers it: an explicit credential-shaped entry has always merged after the
scrub as a deliberate caller opt-in, and an explicit DSH_* entry is the same
deliberate act — the staleness invariant lives entirely in scrubbedParentEnv
dropping AMBIENT credential-shaped and DSH_* names, which stays. The
validation's only observed effect was rejecting legitimate explicit entries:
both recent CI breakages (DSH_GATE_CONCURRENCY exported into every job
crashing lsp specs, DSH_PERMISSION_MODE in acp config.env crashing the
child spawn) were this check firing on values a caller meant to pass, each
fixed by routing around the bureaucracy the seam itself imposed.

The bash seam keeps its own request/spec dshEnv field: that is bash-owned
trusted-plugin vocabulary (the ctx.bashEnv collected overlay) whose merge-last
position guarantees a caller env entry cannot displace a managed fact;
bash-local now flattens ENV_OVERRIDES -> spec.env -> spec.dshEnv into the
seam's one env map. subagent-acp and lsp-local pass their single config env
map straight through. DshEnvironment/DshEnvironmentKey/DSH_ENV_PREFIX stay on
the subprocess seam as the namespace vocabulary (bash re-exports them;
scrubbedParentEnv filters on the prefix).

Tests: the two channel-rejection specs and the splitEnvChannels partition
spec are deleted; one spawn spec now proves an explicit DSH_* env entry
reaches the child while an ambient one is scrubbed; the acp/lsp forwarding
specs keep their MOCK_ECHO_ENV / LSP_FAKE_ECHO_ENV assertions with the split
comments rewritten to merge-after-scrub. Docs (en+zh, re-recorded) and the
owning Agent Notes updated; cordis api/services catalogs regenerated.

docs/core-data-structures/subprocess.md

@@ -8,7 +8,7 @@ Source: [`packages/subprocess/subprocess/src/types.ts`](../../packages/subproces
 
 ## Managed environment namespace and captured output
 
-`DSH_*` variables are Harness-owned child-process facts; implementations discard ambient `DSH_*` names before merging the caller's snapshot, and each collected stream reports its truncation and spill-recovery state through `CollectedOutput`.
+`DSH_*` variables are Harness-owned child-process facts; implementations discard ambient `DSH_*` names before the caller's explicit `env` merges, so a current fact arrives only as a deliberate entry, and each collected stream reports its truncation and spill-recovery state through `CollectedOutput`.
 
 ```ts type-equiv
 /** One environment key inside the managed {@link DSH_ENV_PREFIX} namespace. */
@@ -114,19 +114,14 @@ interface SubprocessSpawnSpec {
    */
   signal?: AbortSignal | undefined
   /**
-   * Ordinary environment entries merged onto the implementation's scrubbed
-   * parent base (see `scrubbedParentEnv`). `DSH_*` names are rejected and
-   * belong in {@link dshEnv}; a deliberately forwarded credential-shaped
-   * entry survives because this layer merges after the scrub.
+   * Explicit environment entries merged onto the implementation's scrubbed
+   * parent base (see `scrubbedParentEnv`), with no namespace validation:
+   * every entry is a deliberate caller opt-in, so a forwarded
+   * credential-shaped entry or a current `DSH_*` fact survives precisely
+   * because this layer merges after the scrub that drops its ambient
+   * namesake.
    */
   env?: Record<string, string> | undefined
-  /**
-   * Harness-owned `DSH_*` variables for this execution. The scrubbed base has
-   * already discarded ambient `DSH_*` entries, so an unavailable current fact
-   * cannot inherit a stale value from the harness process; non-`DSH_*` names
-   * on this channel are rejected.
-   */
-  dshEnv?: DshEnvironment | undefined
 }
 ```