Extract the shared in-process driver into dsh-subagent-inprocess (review feedback)

The shared run driver lived inside dsh-subagent-spawn, so the spawn package
carried fork-aware seeding logic and dsh-subagent-fork depended backward on
dsh-subagent-spawn — the two in-process backends were not independent.

Move the driver (startInProcessRun, depthOf, SubagentDepthError,
InProcessRunOptions) into a new pure-library package
@deepseek-ai/dsh-subagent-inprocess that registers nothing. spawn and fork now
both depend only on that driver and neither knows about the other; spawn no
longer re-exports it and fork no longer imports from spawn.

Also wire BOTH backends in examples/coding-agent/cordis.yml (config-only): load
dsh-subagent-spawn + dsh-subagent-fork + two dsh-tool-subagent instances with
distinct toolNames (subagent → spawn, subagent_fork → fork), demonstrating that
exposing multiple transports needs no code change.

packages/subagent/subagent-spawn/package.json

@@ -20,10 +20,8 @@
   ],
   "license": "BSD-3-Clause",
   "peerDependencies": {
-    "@deepseek-ai/dsh-agent": "^0.0.1",
-    "@deepseek-ai/dsh-llm": "^0.0.1",
-    "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-subagent": "^0.0.1",
+    "@deepseek-ai/dsh-subagent-inprocess": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
   },
   "dependencies": {
@@ -38,6 +36,7 @@
     "@deepseek-ai/dsh-llm-deepseek": "workspace:^",
     "@deepseek-ai/dsh-session": "workspace:^",
     "@deepseek-ai/dsh-subagent": "workspace:^",
+    "@deepseek-ai/dsh-subagent-inprocess": "workspace:^",
     "@deepseek-ai/dsh-system-prompt": "workspace:^",
     "@deepseek-ai/dsh-tool-bash": "workspace:^",
     "@deepseek-ai/dsh-tool-subagent": "workspace:^",