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/snapshots/fs-write-overwrite-bounded/input.json

@@ -0,0 +1,7 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSession" },
+    { "op": "prompt", "text": "First use the read tool to read data.txt in the current directory. Then use the write tool (NOT bash) to replace its entire contents with exactly this single line: The replacement line is deliberately longer than the configured sixty-four byte diff-basis bound. Then reply with exactly the single word DONE." }
+  ]
+}