feat(subprocess): reshape the seam Node-ward for multi-consumer use

Review direction (tianyicui, PR #660): make the interface closer to Node's
API so the other process-running places can adopt it. The spec gains
per-stream stdio dispositions — 'pipe' (raw Readable/Writable for protocol
streams), 'inherit' (diagnostics to the parent), and collect mode ({maxBytes,
spill?} — the old bounded tail-keep shape, now with spill optional for
diagnostic tails). SubprocessOutcome carries exit facts only; collected
output stays readable through handle.collected after settlement (spill fds
are sealed at the settle boundary). The handle grows Node-style kill(signal)
(single signal, tree-scoped, no-op after settlement), terminate() (the
SIGTERM→grace→SIGKILL escalation, also driven by the spec signal),
waitForExit() (tree liveness, not just the direct child), and dispose()
(the cooperative stdin-EOF→SIGTERM→SIGKILL ladder from subagent-subprocess,
graces caller-supplied). Tree semantics are platform-correct: POSIX detached
groups with direct-child fallback; Windows taskkill /T with an injectable
runner. scrubbedParentEnv/SENSITIVE_ENV_PATTERN move to the seam as the one
shared scrub definition.

bash-local maps its config onto collect modes and batch stdin and reads
results through the collected readers; its kill() maps to terminate() so
task_kill keeps escalation semantics.

packages/subprocess/subprocess-local/tests/local.spec.ts

@@ -7,9 +7,11 @@ function spec(command: string, overrides: Partial<SubprocessSpawnSpec> = {}): Su
   return {
     argv: ['bash', '-c', command],
     cwd: process.cwd(),
-    stdoutMaxBytes: 64_000,
-    stderrMaxBytes: 64_000,
-    maxSpillBytes: 64 * 1024 * 1024,
+    stdio: {
+      stdin: 'ignore',
+      stdout: { maxBytes: 64_000, spill: { maxBytes: 64 * 1024 * 1024 } },
+      stderr: { maxBytes: 64_000, spill: { maxBytes: 64 * 1024 * 1024 } },
+    },
     graceMs: 200,
     ...overrides,
   }
@@ -19,9 +21,10 @@ describe('LocalSubprocessService', () => {
   it('registers as ctx.subprocess and spawns managed handles', async () => {
     const ctx = new Context()
     const fiber = await ctx.plugin(LocalSubprocessService)
-    const result = await ctx.subprocess.spawn(spec('echo managed')).done
+    const handle = ctx.subprocess.spawn(spec('echo managed'))
+    const result = await handle.done
     expect(result.exitCode).toBe(0)
-    expect(result.stdout.text).toBe('managed\n')
+    expect(handle.collected.stdout!.readFrom(0).text).toBe('managed\n')
     await fiber.dispose()
   })