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.

docs/module-graph.md

@@ -63,6 +63,10 @@ graph TD
   subagent-mock --> agent
   subagent-mock --> llm
   subagent-mock --> subagent
+  subagent-spawn --> agent
+  subagent-spawn --> llm
+  subagent-spawn --> session
+  subagent-spawn --> subagent
   tool-subagent --> agent
   tool-subagent --> llm
   tool-subagent --> subagent
@@ -75,6 +79,10 @@ graph TD
   stdio-agent --> session
   stdio-agent --> session-persistence-jsonl
   stdio-agent --> ui-stdio
+  subagent-fork --> agent
+  subagent-fork --> session
+  subagent-fork --> subagent
+  subagent-fork --> subagent-spawn
 ```
 
 | Package | Depends on |
@@ -101,6 +109,8 @@ graph TD
 | `tool-bash` | `agent`, `bash`, `llm`, `tools` |
 | `agent-core` | `agent`, `agent-loop`, `invariants`, `llm`, `session`, `system-prompt`, `tool-bash`, `tools` |
 | `subagent-mock` | `agent`, `llm`, `subagent` |
+| `subagent-spawn` | `agent`, `llm`, `session`, `subagent` |
 | `tool-subagent` | `agent`, `llm`, `subagent`, `tools` |
 | `acp-agent` | `acp`, `agent-core`, `session-persistence-jsonl` |
 | `stdio-agent` | `agent`, `agent-core`, `session`, `session-persistence-jsonl`, `ui-stdio` |
+| `subagent-fork` | `agent`, `session`, `subagent`, `subagent-spawn` |