Add in-process subagent backends: spawn (fresh) and fork (seeded)

The second PR of the subagent seam: the two in-process backends that run a
child agent on the same cordis context, reusing the agent factory's quiescent
AgentHandle teardown. Both register on ctx.subagents (PR1's named-provider
registry) and share one run driver.

- dsh-subagent-spawn: a FRESH child via ctx.agents.create — own session, the
  parent's model by default (overridable), zero inherited conversation. Also
  exports the shared in-process run driver (startInProcessRun): mint ids, stamp
  cwd/parentSession-lineage/depth, drive the one-shot (send → whenIdle), read
  the last assistant/message + turn/end reason, dispose to quiescence.
- dsh-subagent-fork: a child SEEDED with the parent's balanced completed-turn
  prefix (the log up to and including its last turn/end), so the child inherits
  context. The in-flight unbalanced turn is excluded — a raw seed would fail the
  invariants replay. Proven: a regression test goes red if the boundary seeds
  the open turn.
- Seam extension: CreateAgentOptions.seed, threaded through AgentLoop.createAgent
  → ctx.sessions.prepare({ seed }) (the primitive resume already used). This is
  the fork-lineage path the TODO(sub-agents) markers anticipated.
- Depth: a merge-extensible AgentOptions.subagentDepth (0 top-level, parent+1 for
  a child); the depthLimit capability refuses a spawn past request.maxDepth.

Tests: real-loop unit tests for both backends (mock MODEL only, real loop +
invariants), a multi-subagent test (one parent drives a fork AND a spawn child
then keeps working), and a with-key e2e (a real parent delegates via the
`subagent` tool to a real child that writes a file on disk — world-verified).
100% per-file coverage. The coding-agent demo wires the spawn backend + tool.

Snapshot coverage of nested agents is deferred to a stacked follow-up
(TODO(subagent-snapshots)): dsh-llm-replay is a single global positional cursor
that cannot route calls to a parent vs. a child on one context. Recorded in the
RFC's deferrals and a new AGENTS.md rule: designing a subsystem must design its
test infrastructure END TO END up front, verifying the snapshot/e2e harness can
express the new shape — a gap this plan hit.

examples/coding-agent/cordis.yml

@@ -48,12 +48,35 @@
     systemPrompt: |
       You are coding-agent, a CLI coding assistant.
 
-      Your only tools are bash (plus bash_output/bash_kill for background
-      tasks). Do ALL file operations through bash: read with cat/sed/head,
-      search with grep, write with heredocs (cat <<'EOF' > file), edit
-      with sed or a rewrite. Each bash call runs in a fresh shell — pass
-      workdir instead of cd, and never rely on shell state between calls.
+      Your tools are bash (plus bash_output/bash_kill for background
+      tasks) and subagent. Do ALL file operations through bash: read with
+      cat/sed/head, search with grep, write with heredocs (cat <<'EOF' >
+      file), edit with sed or a rewrite. Each bash call runs in a fresh
+      shell — pass workdir instead of cd, and never rely on shell state
+      between calls.
+
+      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.
 
       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.)
+- id: subagent
+  name: '@deepseek-ai/dsh-subagent'
+
+- id: subagent-spawn
+  name: '@deepseek-ai/dsh-subagent-spawn'
+  config:
+    providerName: spawn
+
+- id: tool-subagent
+  name: '@deepseek-ai/dsh-tool-subagent'
+  config:
+    provider: spawn