docs(agent-note): flip the regrouping note to implemented

Move the triplet to implemented/architecture/, rewrite Proposal into a
present-tense Decision, fold Acceptance criteria and Risks into
Consequences (recording what pins the result), retitle the migration
section to what the move actually touched, and repoint the seven
inbound FIXME/README links.

packages/credentials/credentials-local/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/credentials/credentials-local/README.md
-README.md: 8e95a890a8e38172cf8984653a01c59570f0061a
-README.zh.md: 04ad07ae4e703ab0416d1d8f1bb6a6ff90adf337
+README.md: 6051c07628c0214f9396d12554719c5614610826
+README.zh.md: c6f3901af925d4bd31d2dcfbf5f5f65de62dac81

packages/credentials/credentials-local/README.md

@@ -51,7 +51,7 @@ External edits publish `credentials/updated` per changed reference after the sna
 
 ## Security boundary
 
-The document is `0600` under a `0700` directory, which stops other OS users — **not** the model. Tool processes (bash, the filesystem tools) run as the same user, and the shipped `workspace-write` file policy confines mutations rather than reads, so they can read this file exactly like any other file the user owns; no sandbox mode singles it out. What the harness does hold to is narrower: it never hands the model a resolved path to the document, and never loads it into the process environment — unlike `$DSH_HOME/.env`, which is the user's ordinary environment layer (see [app-boot's Harness-home layers](../../ui/app-boot/README.md#profiles)) — so reaching the value takes a deliberate read of a path the agent was not given.
+The document is `0600` under a `0700` directory, which stops other OS users — **not** the model. Tool processes (bash, the filesystem tools) run as the same user, and the shipped `workspace-write` file policy confines mutations rather than reads, so they can read this file exactly like any other file the user owns; no sandbox mode singles it out. What the harness does hold to is narrower: it never hands the model a resolved path to the document, and never loads it into the process environment — unlike `$DSH_HOME/.env`, which is the user's ordinary environment layer (see [app-boot's Harness-home layers](../../boot/app-boot/README.md#profiles)) — so reaching the value takes a deliberate read of a path the agent was not given.
 
 That is discretion, not a boundary. A deployment that must keep provider keys away from its own agent cannot get there with file permissions; an OS-keychain provider — a store the model's processes cannot read at all — is the deferred answer and belongs beside this provider as a sibling package.
 

packages/credentials/credentials-local/README.zh.md

@@ -51,7 +51,7 @@ OPENAI_API_KEY: sk-…
 
 ## 安全边界
 
-文档在 `0700` 目录下以 `0600` 权限存放，这挡得住其他 OS 用户，**挡不住**模型。工具进程（bash、文件系统工具）以同一用户身份运行，而已交付的 `workspace-write` 文件策略限制的是修改而非读取，因此它们读这个文件与读该用户拥有的任何其他文件毫无二致；也没有任何沙箱模式会把它单独挑出来。harness 真正守住的更窄：它绝不把该文档的解析后路径交给模型，也绝不把它载入进程环境——这与用户的普通环境层 `$DSH_HOME/.env` 不同（见 [app-boot 的 Harness home 各层](../../ui/app-boot/README.md#profiles)）——因此要拿到这个值，需要刻意去读一条并未交给 agent（智能体）的路径。
+文档在 `0700` 目录下以 `0600` 权限存放，这挡得住其他 OS 用户，**挡不住**模型。工具进程（bash、文件系统工具）以同一用户身份运行，而已交付的 `workspace-write` 文件策略限制的是修改而非读取，因此它们读这个文件与读该用户拥有的任何其他文件毫无二致；也没有任何沙箱模式会把它单独挑出来。harness 真正守住的更窄：它绝不把该文档的解析后路径交给模型，也绝不把它载入进程环境——这与用户的普通环境层 `$DSH_HOME/.env` 不同（见 [app-boot 的 Harness home 各层](../../boot/app-boot/README.md#profiles)）——因此要拿到这个值，需要刻意去读一条并未交给 agent（智能体）的路径。
 
 这是审慎，不是边界。必须让提供方密钥远离自身 agent 的部署无法靠文件权限做到；OS 钥匙串提供方——一个模型的进程根本读不到的存储——才是延后的答案，它应当作为平级包与本提供方并列。