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.

examples/coding-agent/cordis.yml

@@ -57,17 +57,21 @@
 
       Use the subagent tool to delegate a focused, self-contained subtask
       to a fresh child agent (it works in its own context and returns only
-      its final result) — give it a complete, standalone instruction.
+      its final result) — give it a complete, standalone instruction. Use
+      subagent_fork instead when the subtask needs THIS conversation's
+      context: the child inherits the log so far.
 
       Check the [exit code: N] marker on every command; investigate
       failures before moving on. Verify your work by running the code or
       tests. Keep answers brief and factual.
 
-# The subagent seam + an in-process spawn backend + the model-facing `subagent`
-# tool, as leaf entries after the app (which provides ctx.agents/ctx.tools). The
-# tool is bound to the `spawn` backend: a delegated task runs as a fresh child
-# agent on this same process. (fork is available too — load dsh-subagent-fork
-# and a second dsh-tool-subagent bound to it with a distinct toolName.)
+# The subagent seam + BOTH in-process backends + two model-facing tools, as leaf
+# entries after the app (which provides ctx.agents/ctx.tools). spawn (a fresh
+# child) and fork (a child seeded with the parent's completed-turn prefix) are
+# independent backends over the shared dsh-subagent-inprocess driver. Exposing
+# both transports is pure config: load each backend, then load dsh-tool-subagent
+# once per backend with a distinct toolName (the tool registry rejects a
+# duplicate name) — no code change.
 - id: subagent
   name: '@deepseek-ai/dsh-subagent'
 
@@ -76,7 +80,19 @@
   config:
     providerName: spawn
 
+- id: subagent-fork
+  name: '@deepseek-ai/dsh-subagent-fork'
+  config:
+    providerName: fork
+
 - id: tool-subagent
   name: '@deepseek-ai/dsh-tool-subagent'
   config:
     provider: spawn
+    toolName: subagent
+
+- id: tool-subagent-fork
+  name: '@deepseek-ai/dsh-tool-subagent'
+  config:
+    provider: fork
+    toolName: subagent_fork