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.

tsconfig.json

@@ -23,6 +23,7 @@
     { "path": "./vendor/logger-console" },
     { "path": "./packages/util/brand" },
     { "path": "./packages/util/timeout" },
+    { "path": "./packages/util/retention" },
     { "path": "./packages/llm/llm" },
     { "path": "./packages/core/session" },
     { "path": "./packages/session-persistence/session-persistence" },