fix(fs-local): degrade basis I/O failures to null

Descriptor-phase errnos in readTextForDiff fold to before: null so a file
deleted or made unreadable after the caller's preflight cannot fail the
committed write; cancellation and non-errno faults still propagate. Drops
the now-covered isFile v8 ignore, extends llm-replay with catalog
capability parity (defaultMaxTokens/reasoningEfforts), and records the
fs-write-overwrite-bounded keyless snapshot pinning the over-limit
whole-file fallback through the real acp-agent composition.

.agents/notes/implemented/bug-fix/2026-07-30-bounded-overwrite-diff-basis.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 .agents/notes/implemented/bug-fix/2026-07-30-bounded-overwrite-diff-basis.md
-2026-07-30-bounded-overwrite-diff-basis.md: 353b538a12b8cf48dfa3a561c62d6d8ab9a8bfcf
-2026-07-30-bounded-overwrite-diff-basis.zh.md: e2b473a21d16dc47b5b8bf781b8ddffdf443955b
+2026-07-30-bounded-overwrite-diff-basis.md: 7a09934bd1798059de43a092f338d37aa9ccbd9a
+2026-07-30-bounded-overwrite-diff-basis.zh.md: 1d6bdd1068d119aae859132d9f8216ca29d0dc11

.agents/notes/implemented/bug-fix/2026-07-30-bounded-overwrite-diff-basis.md

@@ -10,7 +10,7 @@ English | [中文](2026-07-30-bounded-overwrite-diff-basis.zh.md)
 
 ## Decision
 
-`LocalFileSystem.Config.diffBasisMaxBytes` is a positive safe-integer deployment setting no greater than the runtime's Buffer-allocation and string-decoding limits, with a 10 MiB default. An overwrite supplies `before` only when the UTF-8 replacement is strictly below that limit and the prior file opened for the basis also ends below it. The prior read opens a descriptor, checks that descriptor, and reads at most the configured byte count in cancellation-aware chunks; reaching the boundary returns `null`. A size change after descriptor stat also returns `null`, even if the final size remains below the limit, because a partial prefix would be an incorrect diff basis. Binary or invalid UTF-8 prior content likewise returns `null`. These outcomes do not block the atomic write.
+`LocalFileSystem.Config.diffBasisMaxBytes` is a positive safe-integer deployment setting no greater than the runtime's Buffer-allocation and string-decoding limits, with a 10 MiB default. An overwrite supplies `before` only when the UTF-8 replacement is strictly below that limit and the prior file opened for the basis also ends below it. The prior read opens a descriptor, checks that descriptor, and reads at most the configured byte count in cancellation-aware chunks; reaching the boundary returns `null`. A size change after descriptor stat also returns `null`, even if the final size remains below the limit, because a partial prefix would be an incorrect diff basis. Binary or invalid UTF-8 prior content likewise returns `null`, as does any descriptor-phase errno — a prior file deleted or made unreadable between the caller's preflight and the basis open cannot fail a write the caller already committed to; only cancellation and non-errno faults propagate. These outcomes do not block the atomic write.
 
 The local provider owns this decision because `before` is its optional, best-effort basis: it can avoid acquiring prior content that the configured pair limit has already made ineligible. `tool-fs` continues to own diff computation, retention, and presentation. The setting is independent of `tool-fs.readStreamMinSize`; read routing and overwrite presentation are different policies and need not share a value.
 

.agents/notes/implemented/bug-fix/2026-07-30-bounded-overwrite-diff-basis.zh.md

@@ -10,7 +10,7 @@ Status: implemented
 
 ## Decision
 
-`LocalFileSystem.Config.diffBasisMaxBytes` 是一个不超过运行时 Buffer 分配和字符串解码上限的正安全整数部署配置，默认 10 MiB。只有当 UTF-8 替换内容严格低于该上限，且为生成基础而打开的旧文件最终也低于该上限时，覆写才提供 `before`。旧文件读取会打开文件描述符、检查该描述符，并按可响应取消的分块最多读取配置的字节数；一旦到达边界便返回 `null`。描述符 stat 后发生大小变化时同样返回 `null`，即使最终大小仍低于上限，因为部分前缀会成为错误的 diff 基础。旧内容为二进制或无效 UTF-8 时也返回 `null`。这些结果都不会阻止原子写入。
+`LocalFileSystem.Config.diffBasisMaxBytes` 是一个不超过运行时 Buffer 分配和字符串解码上限的正安全整数部署配置，默认 10 MiB。只有当 UTF-8 替换内容严格低于该上限，且为生成基础而打开的旧文件最终也低于该上限时，覆写才提供 `before`。旧文件读取会打开文件描述符、检查该描述符，并按可响应取消的分块最多读取配置的字节数；一旦到达边界便返回 `null`。描述符 stat 后发生大小变化时同样返回 `null`，即使最终大小仍低于上限，因为部分前缀会成为错误的 diff 基础。旧内容为二进制或无效 UTF-8 时也返回 `null`；描述符阶段的任何 errno 同样如此——旧文件在调用方预检之后、基础读取打开之前被删除或变得不可读，不能让调用方已经提交的写入失败；只有取消和非 errno 故障会继续向上传播。这些结果都不会阻止原子写入。
 
 本地提供方拥有该决策，因为 `before` 是它提供的可选、尽力而为的基础：当配置的成对上限已使替换内容不合格时，它可以避免获取旧内容。`tool-fs` 继续拥有 diff 计算、保留与展示。该配置独立于 `tool-fs.readStreamMinSize`；读取路由与覆写展示是不同策略，无需共享数值。