subagent: implement structured output for in-process backends

The seam vocabulary (SubagentStartRequest.outputSchema, SubagentResult
.structured) existed but no in-process backend honored it — spawn/fork
advertised outputSchema: false. This lands the missing half:

- dsh-tools gains a structured-output JSON Schema subset (json-schema.ts):
  StructuredOutputSchema, assertSupportedOutputSchema (rejects loud outside
  the enforced subset, every violation listed), validateStructuredValue
  (path-qualified issues, total). outputSchema's seam type becomes this raw
  JSON-Schema subset instead of the author-facing SchemaSpec DSL — the schema
  travels verbatim to the model as a forced tool's parameters.
- dsh-subagent-inprocess gains the shared structured runtime: one global
  structured_output capture tool (placeholder parameters) + a prepend:true
  agent/request listener doing FINAL-REQUEST enforcement (strip for plain
  agents, per-run schema for structured children — survives downstream
  request-replacing listeners) + an agent/turn-continuation veto that stops
  a child's turn once captured (no wasted extra model step). Lifetime is
  refcounted by backends (plugin lifetime) AND live runs (start→settle).
- startInProcessRun drives the capture: subset asserted before the child
  exists, instruction appended to the child's system prompt, clean-finish
  nudge loop (structuredNudgeRetries, backend Config, default 1), captured
  value on result.structured; a clean finish without a capture settles
  'error' (never a silent success with a missing field).
- spawn + fork flip outputSchema: true and inject 'tools'.

docs/module-graph.md

@@ -104,6 +104,7 @@ graph TD
   subagent-inprocess --> llm
   subagent-inprocess --> session
   subagent-inprocess --> subagent
+  subagent-inprocess --> tools
   subagent-mock --> agent
   subagent-mock --> llm
   subagent-mock --> subagent
@@ -169,7 +170,7 @@ graph TD
 | `agent-core` | `agent`, `agent-loop`, `invariants`, `llm`, `session`, `system-prompt`, `tool-bash`, `tools` |
 | `hooks-claude` | `agent`, `hook-protocol`, `llm`, `session`, `subagent`, `tools` |
 | `subagent-acp` | `agent`, `llm`, `subagent` |
-| `subagent-inprocess` | `agent`, `llm`, `session`, `subagent` |
+| `subagent-inprocess` | `agent`, `llm`, `session`, `subagent`, `tools` |
 | `subagent-mock` | `agent`, `llm`, `subagent` |
 | `tool-subagent` | `agent`, `llm`, `subagent`, `tools` |
 | `acp-agent` | `acp`, `agent-core`, `app-boot`, `session-persistence-jsonl` |