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

tsconfig.json

@@ -61,6 +61,7 @@
     { "path": "./packages/support/subagent-mock" },
     { "path": "./packages/subagent/tool-subagent" },
     { "path": "./packages/subagent/subagent-inprocess" },
+    { "path": "./packages/subagent/subagent-process" },
     { "path": "./packages/subagent/subagent-spawn" },
     { "path": "./packages/subagent/subagent-fork" },
     { "path": "./packages/subagent/subagent-acp" },