tools/: grouped minor nits - mutable default arg, conint(lt=10000) off-by-one validation crash, dead UC stub, contradictory brighten_hex docs, md5 group ids

Grouped minor findings across the tools/ subsystem (one issue to avoid noise):

1. Mutable default argument - code_puppy/tools/file_modifications.py register_replace_in_file:

def replace_in_file(context: RunContext, file_path: str = "", replacements: RepairableReplacementsList = [], ...)

Classic anti-idiom; pydantic-ai may copy it, but the Python-level default is still a shared list. Use None + normalize, or Field(default_factory=list).

2. ReadFileOutput.num_tokens: conint(lt=10000) off-by-one crash - code_puppy/tools/file_operations.py:40 + _read_file (~line 545): the guard is if num_tokens > 10000: return error, but the model constraint is lt=10000 (strictly less). A file estimating exactly 10000 tokens passes the guard and then fails pydantic validation, raising instead of returning a clean error. Use le=10000 or guard with >=.

3. Dead code - code_puppy/tools/universal_constructor.py:57 _stub_not_implemented() is unused (all actions are implemented now). Also unused imports kept alive: time/ThreadPoolExecutor/FuturesTimeoutError are used, but check subprocess usage is only for ruff - fine; just remove _stub_not_implemented.

4. tools/common.py brighten_hex() docstring says "Darken a hex color" while the function name says brighten and the math int(r * (1 + factor)) brightens for positive factor. Three contradictory signals; fix the docstring.

5. generate_group_id() (tools/common.py, end of file) builds an md5 of timestamp+random for a non-crypto uniqueness token - uuid4().hex[:8] says the same thing in one line without importing hashlib/random locally.

6. code_puppy/tools/file_operations.py _read_file: total_lines counts content.count("\n") of the returned slice when start_line/num_lines are set, but the emitted FileContentMessage.total_lines field name implies whole-file lines - misleading for partial reads.

Filed by Zen Reviewer B (code-puppy-60635a)

Start by reading the named functions in code_puppy/tools/file_modifications.py, code_puppy/tools/file_operations.py, code_puppy/tools/universal_constructor.py, and tools/common.py. Verify each listed cleanup and validation mismatch, then confirm that partial reads report the intended total_lines value and that the existing project tests pass.