refactor(subagent-subprocess): shared provider skeleton for out-of-process backends

The duplication gate flagged three ACP/SDK clones; the shared halves move
into dsh-subagent-subprocess as provider.ts: NO_START_CAPABILITIES (frozen
all-false advertisement), assertPositiveFinite (prefix-parameterized timing
validation), settleRunResult (never-reject result settlement with contained
onError sink and listener hygiene), and subprocessRunHandle (idempotent
dispose publication). Both backends now compose these; the previously
unreachable cancelled-rejection branch is directly unit-tested at the
library level instead of v8-ignored in each backend. knip learns the
subagent-sdk workspace (e2e entry outside vitest unit includes).

packages/subagent/subagent-subprocess/tsconfig.json

@@ -8,6 +8,12 @@
     "src"
   ],
   "references": [
+    {
+      "path": "../../llm/llm"
+    },
+    {
+      "path": "../subagent"
+    },
     {
       "path": "../../support/invariants"
     }