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/composition.md

@@ -43,6 +43,10 @@ flowchart LR
   cfg --> plugin_coding_fs_policy
   plugin_coding_tool_fs["tool-fs<br/>@deepseek-ai/dsh-tool-fs"]
   cfg --> plugin_coding_tool_fs
+  plugin_coding_spill_local["spill-local<br/>@deepseek-ai/dsh-spill-local"]
+  cfg --> plugin_coding_spill_local
+  plugin_coding_spill_policy["spill-policy<br/>@deepseek-ai/dsh-spill-policy"]
+  cfg --> plugin_coding_spill_policy
 ```
 
 | Plugin id | Package / module |
@@ -61,6 +65,8 @@ flowchart LR
 | `fs-local` | `@deepseek-ai/dsh-fs-local` |
 | `fs-policy` | `@deepseek-ai/dsh-fs-policy` |
 | `tool-fs` | `@deepseek-ai/dsh-tool-fs` |
+| `spill-local` | `@deepseek-ai/dsh-spill-local` |
+| `spill-policy` | `@deepseek-ai/dsh-spill-policy` |
 
 Source config: [`examples/coding-agent/cordis.yml`](cordis.yml).