refactor(process): split the process manager out of the bash executor

New process/ capability family: @deepseek-ai/dsh-process owns ctx.processes —
abstract ProcessManager.spawn(spec) over a fully-explicit ProcessSpawnSpec —
plus the shared DSH_* managed-environment and CollectedOutput vocabulary;
@deepseek-ai/dsh-process-local carries the former bash-local run.ts plumbing
(detached groups, tail-keep spill-backed output, credential scrub, kill
escalation, kill-and-join disposal) with no config of its own.

dsh-bash-local becomes a consumer: it keeps command defaulting, the fused
deadline timedOut/aborted classification, the model-friendly terminal env
(now merged through the ordinary env channel), and the [stderr]-marked
background read merge, and spawns through ctx.processes. Background-process
lifetime moves to the manager, so an executor reload no longer kills live
background work; a background spawn failure is injected once into the read
path instead of being buffered as fake stderr. dsh-bash re-exports the moved
vocabulary so bash consumers keep one import root; dsh-bash-sandbox only
redeclares the inherited inject.

Every composition loading a bash executor now loads dsh-process-local (CLI,
examples, python bundled runtime, create-sdk bash feature, inline test
configs).

packages/bash/bash-sandbox/tests/sandbox.spec.ts

@@ -15,6 +15,7 @@ import { SANDBOX_UNAVAILABLE, SandboxProvider, SandboxUnavailableError } from '@
 import type { ConfinedArgv, SandboxExecutionPolicy, SandboxMode, SandboxPolicy } from '@deepseek-ai/dsh-sandbox'
 import { SandboxPolicyService } from '@deepseek-ai/dsh-sandbox-policy'
 import { SandboxBashExecutor } from '@deepseek-ai/dsh-bash-sandbox'
+import LocalProcessManager from '@deepseek-ai/dsh-process-local'
 import { classifyDenial, classifyRunnerFailure, shellQuote } from '../src/helpers.ts'
 import type { Config } from '@deepseek-ai/dsh-bash-sandbox'
 
@@ -58,9 +59,10 @@ async function setup(
     ...mode !== undefined ? { mode } : {},
     ...workspaceRoot !== undefined ? { workspaceRoot } : {},
   })
+  await ctx.plugin(LocalProcessManager)
+  ;(ctx.processes as LocalProcessManager).internals = { spillDir }
   await ctx.plugin(SandboxBashExecutor, { graceMs: 200, ...execConfig })
   const bash = ctx.bash as SandboxBashExecutor
-  bash.internals = { spillDir }
   return { ctx, bash, calls }
 }