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.

examples/acp-agent/tests/fs-diff-bound.cordis.yml

@@ -0,0 +1,29 @@
+# Live counterpart for the bounded-overwrite-diff snapshot: the base stack with
+# the fs backend's overwrite diff-basis limit shrunk so a modest replacement
+# crosses the exclusive bound and the write result falls back to a whole-file
+# diff. A config patch replaces the row's whole config, so `cwd` is restated
+# verbatim, and the acp-agent restatement re-pins `deepseek-v4-flash` to match
+# the recorded corpus and its pinned request headers.
+- id: base
+  name: '@cordisjs/plugin-include'
+  config:
+    path: ../cordis.yml
+    patches:
+      - id: acp-agent
+        name: '@deepseek-ai/dsh-acp-demo'
+        config:
+          provider: deepseek-official
+          model: deepseek-v4-flash
+          persistenceRoot: !!js process.env.DSH_SNAPSHOT_SESSIONS_ROOT ?? './.sessions'
+          persistenceCompression: !!js "process.env.DSH_SNAPSHOT === undefined ? 'zstd' : 'none'"
+          workspaceContext:
+            maxBytes: 65536
+          persona: |
+            You are a coding assistant powered by the {{model}} model. Your working directory is {{cwd}}. Your bash tool runs under a file sandbox — a `[sandbox: file access denied …]` result is policy, not a command bug.
+
+            Verify your work by running the code or tests. Keep answers brief and factual.
+      - id: fs-sandbox
+        name: '@deepseek-ai/dsh-fs-sandbox'
+        config:
+          cwd: !!js process.cwd()
+          diffBasisMaxBytes: 64