Configure Additional settings.json Options for Claude Code

## Context
Following up on #564 and #577, we need to configure the remaining `settings.json` options that aren't yet managed as code.

## Settings to Configure

### 1. `cleanupPeriodDays` (Chat Transcript Retention)
- **Current**: Default 30 days
- **Desired**: 45 days
- **Reason**: Keep chat history longer for reference

### 2. `includeCoAuthoredBy` (Git Commit Byline)
- **Current**: Default true (includes "Co-Authored-By: Claude")
- **Desired**: false
- **Reason**: User preference to not include co-author attribution

### 3. `apiKeyHelper` (Optional)
- Script to generate auth values dynamically
- May be useful for rotating credentials

## Proposed Implementation

Add to either:
1. `.claude/settings/claude-code-defaults.json` if supported by `claude config set`
2. `~/.claude/settings.json` for user-level settings
3. `.claude/settings.local.json` for project-level

Example configuration:
```json
{
"cleanupPeriodDays": 45,
"includeCoAuthoredBy": false
}
```

## Testing
- Verify chat transcripts are retained for 45 days
- Confirm git commits don't include Co-Authored-By line
- Check if `claude config set` supports these keys

## Benefits
- Longer chat history retention for debugging/reference
- Cleaner git commits without bot attribution
- All preferences survive the "spilled coffee test"

Related: #577, #579
Principle: systems-stewardship

Review .claude/settings/claude-code-defaults.json, ~/.claude/settings.json, and .claude/settings.local.json, then check whether claude config set supports the named keys. Verify 45-day transcript retention and commits without the co-author line; decide how apiKeyHelper should be handled before considering the work done.

Written by the indexing model from the issue text.