Merge origin/master: port the SDK stack onto the subprocess seam

Master's #660 replaced dsh-subagent-subprocess with the dsh-subprocess
capability seam (ctx.subprocess + scrubbedParentEnv, tree-scoped teardown)
and moved subagent-acp onto it. Convergence for this branch's packages:

- The shared out-of-process provider vocabulary this branch had grown in
  the deleted library (NO_START_CAPABILITIES, assertPositiveFinite, cwd
  resolution, settleRunResult, subprocessRunHandle) moves into the subagent
  seam package as out-of-process.ts — it enforces subagent-seam contracts,
  not process mechanics, and both out-of-process backends now import it
  from there (subagent-acp keeps master's shape otherwise).
- subagent-sdk spawns THROUGH the SDK client (the subprocess README's
  documented exception for SDK-managed transports) and now applies the
  seam's scrubbedParentEnv() + explicit-env merge in place of the deleted
  buildChildEnv.
- sdk-client inlines the EOF→SIGTERM→SIGKILL ladder as private helpers (it
  runs outside any harness context, so it cannot ride ctx.subprocess).
- The child harness fixture gains the now-required dsh-subprocess-local
  entry for bash-local; the fixture cordis.yml keeps exercising the
  shipped provider default.

knip.json

@@ -6,13 +6,15 @@
   "ignoreBinaries": [
     "bwrap",
     "python3",
-    "sandbox-exec"
+    "sandbox-exec",
+    "taskkill"
   ],
   "ignoreWorkspaces": [
     "vendor/*",
     "python/sdk-runtime"
   ],
   "ignoreDependencies": [
+    "@yarnpkg/cli-dist",
     "lightningcss"
   ],
   "workspaces": {
@@ -503,15 +505,6 @@
         "tests/**/*.ts"
       ]
     },
-    "packages/subagent/subagent-subprocess": {
-      "entry": [
-        "tests/**/*.spec.ts"
-      ],
-      "project": [
-        "src/**/*.ts",
-        "tests/**/*.ts"
-      ]
-    },
     "packages/fs/tool-fs": {
       "entry": [
         "tests/**/*.spec.ts",