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.

tsconfig.build.json

@@ -35,6 +35,7 @@
     { "path": "./packages/subagent/subagent" },
     { "path": "./packages/support/subagent-mock" },
     { "path": "./packages/subagent/tool-subagent" },
+    { "path": "./packages/subagent/subagent-inprocess" },
     { "path": "./packages/subagent/subagent-spawn" },
     { "path": "./packages/subagent/subagent-fork" }
   ]