feat(retention): add dsh-retention bounded-output library

Ship @deepseek-ai/dsh-retention under packages/util/: pure ItemRetainer /
TextRetainer plus neutral notice helpers, so tools that cap model-facing
output share one "what did we keep, what did we omit, may we stop reading"
mechanic while keeping grouping, exit codes, provider errors, and recovery
prose tool-owned. The two retainers are separate names because they differ in
resource model: item-head can stop the upstream on the first over-cap probe
(shouldStop), while text tail/head-tail must read to the end. The library
documents glob/grep/bash/web_fetch/web_search mappings but migrates no tool
yet — glob/grep don't exist, and migration is deliberately separate work.

Flips the RFC to implemented/ and rewrites its skeleton to shipped reality.

packages/util/retention/package.json

@@ -0,0 +1,30 @@
+{
+  "name": "@deepseek-ai/dsh-retention",
+  "description": "Zero-dependency bounded-retention primitive: ItemRetainer/TextRetainer + neutral notice helpers (what did we keep, what did we omit, may the caller stop reading)",
+  "version": "0.0.1",
+  "private": true,
+  "type": "module",
+  "main": "lib/index.js",
+  "types": "lib/types/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./lib/types/index.d.ts",
+      "default": "./lib/index.js"
+    },
+    "./src/*": "./src/*",
+    "./package.json": "./package.json"
+  },
+  "files": [
+    "lib/index.js",
+    "lib/types/**/*.d.ts",
+    "lib/types/**/*.d.ts.map",
+    "src"
+  ],
+  "license": "BSD-3-Clause",
+  "peerDependencies": {
+    "cordis": "^4.0.0-rc.6"
+  },
+  "devDependencies": {
+    "cordis": "^4.0.0-rc.6"
+  }
+}