feat(spill): add tool-output spill seam, local backend, and policy

Oversized plain-text tool results now spill to a session-scoped file and
return a bounded preview plus the spill path, so a verbose result stays
readable via `read` without consuming the next model request in full.

- dsh-spill: minimal SpillFiles seam (saveText → session-scoped SpillPath)
- dsh-spill-local: private 0700 session dirs, traversal-safe names, exclusive
  owner-only writes
- dsh-spill-policy: tools/post-execute transformer; no-op unless maxInlineBytes
  is set; skips read; best-effort on save failure (never turns a success into
  an isError)

web_fetch is the showcase — no tool-specific spill code. The coding-agent
example loads the stack so its keyless Loader smoke guards the namespace-plugin
export shape. Snapshot gap for a transcript-visible web_fetch spill is recorded
in the RFC's Consequences (ACP replay is keyless and cannot hit the web).

scripts/gen-doc-graphs.ts

@@ -74,6 +74,7 @@ const GROUP_ORDER = [
   'compact',
   'subagent',
   'web',
+  'spill',
   'todo',
   'hooks',
   'session-persistence',
@@ -186,6 +187,15 @@ const SERVICE_ROLES: ServiceRole[] = [
     consumers: ['tool-web'],
     note: 'Search and fetch providers register into one ctx.web seam; tool-web owns the stable model-facing names.',
   },
+  {
+    key: 'spillFiles',
+    pkg: 'spill',
+    title: 'Spill storage seam',
+    mode: 'seam',
+    implementations: ['spill-local'],
+    consumers: ['spill-policy'],
+    note: 'The backend saves oversized tool text to a session-scoped path; spill-policy is the tools/post-execute consumer that decides when to spill.',
+  },
 ]
 
 const DYNAMIC_EVENT_DISPATCHERS: Array<{ event: string; pkg: string; method: string }> = [