Implement waza tokens compare command (microsoft#111)

Adds a new `tokens compare` subcommand that compares token counts in a
directory between git refs, enabling users to track how token usage
changes across commits or between committed and uncommitted work. Closes
microsoft#51

## Usage

```bash
# Compare HEAD to working tree
waza tokens compare

# Compare a specific ref to working tree
waza tokens compare HEAD~3

# Compare two refs
waza tokens compare main feature-branch

# JSON output
waza tokens compare --format json

# Include unchanged files
waza tokens compare --show-unchanged
```

## Changes

### New files

| File | Description |
| -------------------------------------------- |
-----------------------------------------------------------------------------------------------------------
|
| `cmd/waza/tokens/compare.go` | Command implementation: ref resolution,
file diffing, token counting, table and JSON output |
| `cmd/waza/tokens/compare_test.go` | Tests covering
added/modified/removed files, multiple ref combinations,
`--show-unchanged`, and JSON output |
| `cmd/waza/tokens/internal/git/git.go` | Git helpers: `IsInRepo`,
`GetFilesFromRef`, `GetFileFromRef`, `RefExists` |

## Design

- **Ref handling:** With no args, compares `HEAD → WORKING` i.e., the
working tree. One arg compares that ref to working tree. Two args
compares first to second.
- **File detection:** Uses `git ls-files` for the working tree and `git
ls-tree` for committed refs, filtering to `.md`/`.mdx` files.
- **Edge cases:** Handles empty repos (no HEAD), deleted working tree
files, and files that exist in only one ref.
- **Output formats:** Table (default) with emoji status indicators, or
structured JSON with per-file and summary statistics.
- **Per-file stats:** Token count, character count, line count, absolute
diff, percent change, and status
(`added`/`removed`/`modified`/`unchanged`).

## Example Output
```
📊 Token Comparison: HEAD → WORKING
File                  Before     After      Diff  Status
------------------------------------------------------------------
README.md                 11        14        +3  📈
references/spec.md         7         7         0  ➡️
unchanged.md               1         1         0  ➡️
------------------------------------------------------------------
Total                     19        22        +3  15.8%
📋 Summary:
    Added: 0, Removed: 0, Modified: 1
    Increased: 1, Decreased: 0
```

Author: chlowell