feat(subagent): extract dsh-subagent-process shared out-of-process machinery

The credential env scrub (SENSITIVE_ENV_PATTERN/buildChildEnv), the
spawn-failure capture (spawnFailure), the child-exit waits
(waitForExit/exitsWithin), and the stdin-EOF -> SIGTERM -> SIGKILL dispose
ladder move out of subagent-acp into a new pure library package (the
subagent-inprocess shape), with the ladder taking its two grace periods as
parameters — defaults stay in the plugin Config. New isolated-config-dir
helpers (mkdtemp create, best-effort remove; a pinned dir is never removed)
land alongside for the CLAUDE_CONFIG_DIR / CODEX_HOME redirection the RFC
names.

The ACP backend migrates onto the library with no semantic change: its suite
passes with import-path edits only. bash-local keeps its sibling copy, per
the RFC's blast-radius call.

RFC: docs/rfc/proposed/feature/2026-07-07-claude-code-and-codex-subagent-backends.md

knip.json

@@ -66,6 +66,11 @@
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts", "tests/mock-acp-server.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"]
     },
+    "packages/subagent/subagent-process": {
+      "entry": ["tests/**/*.spec.ts"],
+      "project": ["src/**/*.ts", "tests/**/*.ts"],
+      "ignoreDependencies": ["cordis"]
+    },
     "packages/fs/tool-fs": {
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"]