feat(file_tools): harden read_file with size guard, dedup, and device blocking (#4315)

* feat(file_tools): harden read_file with size guard, dedup, and device blocking

Three improvements to read_file_tool to reduce wasted context tokens and
prevent process hangs:

1. Character-count guard: reads that produce more than 100K characters
   (≈25-35K tokens across tokenisers) are rejected with an error that
   tells the model to use offset+limit for a smaller range.  The
   effective cap is min(file_size, 100K) so small files that happen to
   have long lines aren't over-penalised.  Large truncated files also
   get a hint nudging toward targeted reads.

2. File-read deduplication: when the same (path, offset, limit) is read
   a second time and the file hasn't been modified (mtime unchanged),
   return a lightweight stub instead of re-sending the full content.
   Writes and patches naturally change mtime, so post-edit reads always
   return fresh content.  The dedup cache is cleared on context
   compression — after compression the original read content is
   summarised away, so the model needs the full content again.

3. Device path blocking: paths like /dev/zero, /dev/random, /dev/stdin
   etc. are rejected before any I/O to prevent process hangs from
   infinite-output or blocking-input devices.

Tests: 17 new tests covering all three features plus the dedup-reset-
on-compression integration.  All 52 file-read tests pass (35 existing +
17 new).  Full tool suite (2124 tests) passes with 0 failures.

* feat: make file_read_max_chars configurable, add docs

Add file_read_max_chars to DEFAULT_CONFIG (default 100K).  read_file_tool
reads this on first call and caches for the process lifetime.  Users on
large-context models can raise it; users on small local models can lower it.

Also adds a 'File Read Safety' section to the configuration docs
explaining the char limit, dedup behavior, and example values.

website/docs/user-guide/configuration.md

@@ -360,6 +360,26 @@ memory:
   user_char_limit: 1375     # ~500 tokens
 ```
 
+## File Read Safety
+
+Controls how much content a single `read_file` call can return. Reads that exceed the limit are rejected with an error telling the agent to use `offset` and `limit` for a smaller range. This prevents a single read of a minified JS bundle or large data file from flooding the context window.
+
+```yaml
+file_read_max_chars: 100000  # default — ~25-35K tokens
+```
+
+Raise it if you're on a model with a large context window and frequently read big files. Lower it for small-context models to keep reads efficient:
+
+```yaml
+# Large context model (200K+)
+file_read_max_chars: 200000
+
+# Small local model (16K context)
+file_read_max_chars: 30000
+```
+
+The agent also deduplicates file reads automatically — if the same file region is read twice and the file hasn't changed, a lightweight stub is returned instead of re-sending the content. This resets on context compression so the agent can re-read files after their content is summarized away.
+
 ## Git Worktree Isolation
 
 Enable isolated git worktrees for running multiple agents in parallel on the same repo: