Introduces a new `hk util` subcommand for utility operations, starting with
trailing-whitespace detection and fixing.

## Features

- **hk util trailing-whitespace <files>** - Check for trailing whitespace
- **hk util trailing-whitespace --fix <files>** - Remove trailing whitespace
- Automatically skips non-text files
- Exit code 1 if issues found/fixed, 0 if clean

## Implementation

- New `src/cli/util.rs` module with extensible subcommand structure
- Rust implementation replaces bash scripts for better:
  - Testability (unit tests in Rust)
  - Readability (no complex bash escaping)
  - Cross-platform compatibility
- `trailing_whitespace` builtin now calls `hk util trailing-whitespace`

## Testing

**Rust unit tests** (4 tests in src/cli/util.rs):
- test_has_trailing_whitespace
- test_no_trailing_whitespace
- test_fix_trailing_whitespace
- test_fix_already_clean

**Bats integration tests** (9 tests in test/util_trailing_whitespace.bats):
- Detection of trailing whitespace
- Fixing trailing whitespace
- Multiple files support
- Binary file skipping
- Various whitespace types (space, tab, mixed)
- Builtin integration with hk check/fix

## Example Usage

```bash
# Check files
hk util trailing-whitespace file1.txt file2.txt

# Fix files
hk util trailing-whitespace --fix *.txt

# Via builtin
hk.pkl:
  hooks {
    ["check"] {
      steps {
        ["trailing-ws"] = Builtins.trailing_whitespace
      }
    }
  }
```

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Co-Authored-By: Claude <[email protected]>

Replace unix-specific 'file' command with cross-platform Rust implementation.

## Changes

- Replace `file -b --mime-type` with pure Rust heuristics:
  - Read first 8KB of file
  - Check for null bytes (binary indicator)
  - Validate UTF-8 encoding
  - Empty files treated as text

## Benefits

- ✅ Works on Windows (no dependency on 'file' command)
- ✅ Works on macOS and Linux
- ✅ Faster (no subprocess spawn)
- ✅ More consistent behavior across platforms

## Testing

Added 3 new unit tests:
- test_is_text_file_with_text
- test_is_text_file_with_binary
- test_is_text_file_with_empty

Total: 7 Rust unit tests

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Co-Authored-By: Claude <[email protected]>

Fixes docs-sync CI failure by adding missing documentation.

## Changes

- **docs/builtins.md**: Add trailing_whitespace entry in Special Purpose section
- **docs/cli/util.md**: New file documenting hk util command
- **docs/cli/index.md**: Add util subcommand to CLI reference

## Details

### trailing_whitespace builtin
- Files: All text files
- Features: Detect and remove trailing whitespace
- Commands: hk util trailing-whitespace [--fix] {{files}}
- Cross-platform Rust implementation

### util command
- New utility command framework
- Starting with trailing-whitespace subcommand
- Designed for extensibility (future: end-of-file, mixed-line-ending, etc.)

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Co-Authored-By: Claude <[email protected]>

The builtin integration test was failing because it tried to import
Builtins.trailing_whitespace from the released package (v1.2.0), which
doesn't have this new builtin yet.

## Change

Use local pkl files via $PKL_PATH instead of package imports:
- amends "$PKL_PATH/Config.pkl"
- import "$PKL_PATH/Builtins.pkl"

This allows the test to use the newly created trailing_whitespace builtin
that's being added in this PR.

## Fix

Resolves CI failure in ubuntu tests:
```
Cannot find property `trailing_whitespace` in module `Builtins`.
```

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Co-Authored-By: Claude <[email protected]>

Changes:
- Fix bats test PATH to respect CARGO_TARGET_DIR env var (set by mise)
- Change fix mode to exit 0 on success (standard formatter behavior)
- Fix builtin test expectations to use 'code' instead of 'status'
- Add 'fix' hook to builtin integration test

This resolves the "hk: command not found" issue when running mise run ci
locally and ensures the utility works correctly in hk fix hooks.

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Co-Authored-By: Claude <[email protected]>

Fixes the eslint step in hk check which uses 'bun run eslint'.
Also ran 'bun install' to install all node dependencies.

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Co-Authored-By: Claude <[email protected]>