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/cordis-catalog/services.md

@@ -165,6 +165,22 @@ list(): Session[]
 
 Source: [`packages/core/session/src/index.ts:327`](../../packages/core/session/src/index.ts)
 
+## `ctx.spillFiles` — `SpillFiles` (abstract seam)
+
+Abstract spill storage service. Subclass, implement saveText, and load the subclass as a plugin — it registers as `ctx.spillFiles` (one implementation per context; loading a second throws, cordis' standard duplicate-service behavior).
+
+Semantics every implementation must honor:
+
+- saveText persists the FULL `content` verbatim and returns a path the local `read` tool can open, plus the exact byte length written.
+- Storage is scoped by the request's SaveTextSpill.owner session; the backend chooses a private (not world-readable) location and a collision-free name derived from — never equal to — the caller's `suggestedName`.
+- `saveText` REJECTS on a real storage failure (permissions, ENOSPC, backend unavailable); the caller decides how to degrade (the spill policy treats a rejection as best-effort and keeps the inline result).
+
+```ts cordis-catalog
+abstract saveText(input: SaveTextSpill): Promise<SpillRef>
+```
+
+Source: [`packages/spill/spill/src/index.ts:46`](../../packages/spill/spill/src/index.ts)
+
 ## `ctx.subagents` — `SubagentService`
 
 The `subagents` service: a registry of named SubagentProviders and a capability-checked start surface.