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).

examples/coding-agent/cordis.yml

@@ -120,3 +120,16 @@
 
 - id: tool-fs
   name: '@deepseek-ai/dsh-tool-fs'
+
+# Tool-output spill stack: a local backend that saves oversized tool text under
+# a private session-scoped dir, and the tools/post-execute policy that replaces
+# an over-budget plain-text result with a preview + the spill path (the model
+# reads the full result later). A leaf pair after the app (needs ctx.tools). The
+# policy is a no-op until a tool returns more than maxInlineBytes of plain text.
+- id: spill-local
+  name: '@deepseek-ai/dsh-spill-local'
+
+- id: spill-policy
+  name: '@deepseek-ai/dsh-spill-policy'
+  config:
+    maxInlineBytes: 50000