🤖 Add byte limits to bash tool output (#91)

## Problem

The bash tool can break models due to excessive output despite the
1000-line hard cap:
- Lines can be arbitrarily long (1000 lines × 100KB/line = 100MB
possible)
- No byte-based limits exist
- Truncated partial output pollutes context and doesn't teach models to
be conservative

## Solution

Add byte limits that **fail with an error instead of returning truncated
output**:

- **BASH_MAX_LINE_BYTES**: 1KB per line
- **BASH_MAX_TOTAL_BYTES**: 16KB total output
- **Commands that exceed limits fail completely** (no partial output)

This encourages models to be conservative and use output-limiting
commands.

### Changes

1. **Constants** (`src/constants/toolLimits.ts`):
   - Added `BASH_MAX_LINE_BYTES = 1024`
   - Added `BASH_MAX_TOTAL_BYTES = 16 * 1024`

2. **Bash Tool** (`src/services/tools/bash.ts`):
   - Track `totalBytesAccumulated` across stdout/stderr
   - Kill process when per-line or total byte limits exceeded
   - Return error (not partial output) when limits hit
   - Error message suggests using head/tail/grep

3. **Tool Definition** (`src/utils/tools/toolDefinitions.ts`):
   - Emphasize limits are strict and will FAIL
   - Encourage conservative usage: "use head, tail, grep"
   - Clear messaging that no partial output is returned

4. **Tests** (`src/services/tools/bash.test.ts`):
   - Test per-line limit fails correctly
   - Test total bytes limit fails correctly
   - Test early termination returns error
   - Verify error messages include helpful suggestions

## Benefits

- **Forces conservative behavior**: Models learn to limit output
proactively
- **Clean context**: No polluted partial output in conversation history
- **Predictable failures**: Clear error messages guide models to fix
commands
- **Early termination**: Don't waste cycles processing excessive output

_Generated with `cmux`_

Author: ammario

src/constants/toolLimits.ts

@@ -1,2 +1,4 @@
 export const BASH_DEFAULT_MAX_LINES = 1000;
 export const BASH_HARD_MAX_LINES = 1000;
+export const BASH_MAX_LINE_BYTES = 1024; // 1KB per line
+export const BASH_MAX_TOTAL_BYTES = 16 * 1024; // 16KB total output