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).

docs/module-graph.md

@@ -56,6 +56,11 @@ flowchart TD
     pkg_web_search_exa["web-search-exa"]
     pkg_web_search_perplexity["web-search-perplexity"]
   end
+  subgraph group_spill["packages/spill"]
+    pkg_spill["spill"]
+    pkg_spill_local["spill-local"]
+    pkg_spill_policy["spill-policy"]
+  end
   subgraph group_todo["packages/todo"]
     pkg_tool_todo["tool-todo"]
   end
@@ -105,6 +110,9 @@ flowchart TD
   pkg_web_search_deepseek --> pkg_web
   pkg_web_search_exa --> pkg_web
   pkg_web_search_perplexity --> pkg_web
+  pkg_spill --> pkg_brand
+  pkg_spill --> pkg_llm
+  pkg_spill --> pkg_session
   pkg_hook_protocol --> pkg_bash
   pkg_hook_protocol --> pkg_session
   pkg_session_persistence --> pkg_session
@@ -117,6 +125,7 @@ flowchart TD
   pkg_compact_basic --> pkg_compact
   pkg_compact_basic --> pkg_llm
   pkg_compact_basic --> pkg_session
+  pkg_spill_local --> pkg_spill
   pkg_session_persistence_jsonl --> pkg_session
   pkg_session_persistence_jsonl --> pkg_session_persistence
   pkg_session_persistence_sqlite --> pkg_session
@@ -147,6 +156,11 @@ flowchart TD
   pkg_tool_web --> pkg_system_prompt
   pkg_tool_web --> pkg_tools
   pkg_tool_web --> pkg_web
+  pkg_spill_policy --> pkg_llm
+  pkg_spill_policy --> pkg_retention
+  pkg_spill_policy --> pkg_session
+  pkg_spill_policy --> pkg_spill
+  pkg_spill_policy --> pkg_tools
   pkg_tool_todo --> pkg_agent
   pkg_tool_todo --> pkg_session
   pkg_tool_todo --> pkg_tools
@@ -229,11 +243,13 @@ flowchart TD
 | [`web-search-deepseek`](../packages/web/web-search-deepseek) | `web` | [`web`](../packages/web/web) |
 | [`web-search-exa`](../packages/web/web-search-exa) | `web` | [`web`](../packages/web/web) |
 | [`web-search-perplexity`](../packages/web/web-search-perplexity) | `web` | [`web`](../packages/web/web) |
+| [`spill`](../packages/spill/spill) | `spill` | [`brand`](../packages/util/brand), [`llm`](../packages/llm/llm), [`session`](../packages/core/session) |
 | [`hook-protocol`](../packages/hooks/hook-protocol) | `hooks` | [`bash`](../packages/bash/bash), [`session`](../packages/core/session) |
 | [`session-persistence`](../packages/session-persistence/session-persistence) | `session-persistence` | [`session`](../packages/core/session) |
 | [`llm-replay`](../packages/support/llm-replay) | `support` | [`llm`](../packages/llm/llm), [`session`](../packages/core/session) |
 | [`tools`](../packages/core/tools) | `core` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`system-prompt`](../packages/core/system-prompt) |
 | [`compact-basic`](../packages/compact/compact-basic) | `compact` | [`agent`](../packages/core/agent), [`compact`](../packages/compact/compact), [`llm`](../packages/llm/llm), [`session`](../packages/core/session) |
+| [`spill-local`](../packages/spill/spill-local) | `spill` | [`spill`](../packages/spill/spill) |
 | [`session-persistence-jsonl`](../packages/session-persistence/session-persistence-jsonl) | `session-persistence` | [`session`](../packages/core/session), [`session-persistence`](../packages/session-persistence/session-persistence) |
 | [`session-persistence-sqlite`](../packages/session-persistence/session-persistence-sqlite) | `session-persistence` | [`session`](../packages/core/session), [`session-persistence`](../packages/session-persistence/session-persistence) |
 | [`invariants`](../packages/support/invariants) | `support` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`session`](../packages/core/session) |
@@ -242,6 +258,7 @@ flowchart TD
 | [`tool-fs`](../packages/fs/tool-fs) | `fs` | [`fs`](../packages/fs/fs), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools) |
 | [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`tools`](../packages/core/tools) |
 | [`tool-web`](../packages/web/tool-web) | `web` | [`llm`](../packages/llm/llm), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools), [`web`](../packages/web/web) |
+| [`spill-policy`](../packages/spill/spill-policy) | `spill` | [`llm`](../packages/llm/llm), [`retention`](../packages/util/retention), [`session`](../packages/core/session), [`spill`](../packages/spill/spill), [`tools`](../packages/core/tools) |
 | [`tool-todo`](../packages/todo/tool-todo) | `todo` | [`agent`](../packages/core/agent), [`session`](../packages/core/session), [`tools`](../packages/core/tools) |
 | [`hooks-codex`](../packages/hooks/hooks-codex) | `hooks` | [`agent`](../packages/core/agent), [`hook-protocol`](../packages/hooks/hook-protocol), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`tools`](../packages/core/tools) |
 | [`acp`](../packages/ui/acp) | `ui` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`session-persistence`](../packages/session-persistence/session-persistence), [`tools`](../packages/core/tools) |