fix(kernel,tools,validation): unblock role_pipeline end-to-end + QA audit

Live QA audit of role_pipeline against a sample repo surfaced and fixed a
chain of defects that prevented any tool+artifact workflow from running
end-to-end. The pipeline now completes cleanly with a real, validated,
reviewed file change.

- F-008 workflow: propagate stage token_budget -> generationConfig.maxTokens
  (TomlWorkflowLoader) so large stages stop truncating at the 2048 default.
- F-009 tools/kernel: file_read missing-file is recoverable; recoverable tool
  errors feed back into the loop instead of aborting the stage.
- F-010/F-018 kernel: reject premature stage_complete and nudge the model to
  emit the owed artifact / invoke a write tool (bounded by MAX_TOOL_ROUNDS).
- F-011 schemas: list-typed artifact fields string -> array (analysis/design/
  impl_plan) to match model output.
- F-012 kernel: strip an outer markdown code fence from LLM artifacts.
- F-013 validation: payload-validation failures are retryable; structural
  failures stay terminal (wires the invariant-#7 reject+retry contract).
- F-014 tools: file_edit schema matches its real params; edits emit a diff.
- F-015 kernel: record tool-execution events + materialise a real file_written
  artifact from the on-disk write; gate validation so a no-write stage cannot
  be rubber-stamped (no more false-success runs).
- F-016 server: raise stageTimeoutMs 60s -> 180s for slow local models.
- F-019 artifacts/kernel: file_written artifact carries the unified diff so the
  reviewer can verify the change (shared DiffUtil; file_write emits a diff too).
- F-020 tools: model-correctable file_edit failures are recoverable + steer
  toward replace/file_write over the fragile patch op.

Full findings register (F-001..F-021, incl. open F-021 resume rehydrate bug)
in qa/audit-report-2026-06-08.md. All module tests green.

core/artifacts/src/main/kotlin/com/correx/core/artifacts/kind/FileWrittenKind.kt

@@ -10,6 +10,7 @@ object FileWrittenKind : ArtifactKind {
             "path" to JsonSchemaProperty(type = "string", description = "Relative file path, no .. components"),
             "content" to JsonSchemaProperty(type = "string", description = "File content"),
             "mode" to JsonSchemaProperty(type = "string", description = "Unix file mode", default = "0644"),
+            "diff" to JsonSchemaProperty(type = "string", description = "Unified diff of the change"),
         ),
         required = listOf("path", "content"),
         additionalProperties = false,

core/artifacts/src/main/kotlin/com/correx/core/artifacts/kind/FileWrittenPayload.kt

@@ -15,4 +15,7 @@ data class FileWrittenArtifact(
     val contentHash: String,
     val size: Long,
     val mode: String,
+    // Unified diff of the change, so downstream consumers (e.g. the reviewer stage) can see WHAT
+    // changed rather than only metadata. Null/absent when no diff was captured.
+    val diff: String? = null,
 )