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/cordis-catalog/events.md

@@ -287,7 +287,7 @@ A tool was registered or unregistered (the available tool set changed).
 'tools/change'(): void
 ```
 
-Source: [`packages/core/tools/src/index.ts:87`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:97`](../../packages/core/tools/src/index.ts)
 
 ### `tools/post-execute` — waterfall
 
@@ -299,7 +299,7 @@ Waterfall AFTER a tool runs — where hook plugins inspect the result and accept
 
 Types: [ToolExecution](../core-data-structures/tools.md) · [ToolExecutionResult](../core-data-structures/tools.md)
 
-Source: [`packages/core/tools/src/index.ts:82`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:92`](../../packages/core/tools/src/index.ts)
 
 ### `tools/pre-execute` — waterfall
 
@@ -311,7 +311,7 @@ Waterfall BEFORE a tool runs — the gate where sandbox, permission, and hook pl
 
 Types: [ToolExecution](../core-data-structures/tools.md)
 
-Source: [`packages/core/tools/src/index.ts:66`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:76`](../../packages/core/tools/src/index.ts)
 
 ## Inherited events (cordis core + loader/hmr/timer)