Enhancement: Handle bot/app-authored PRs in conflict detection

## Problem

When a detected conflict involves a PR authored by a bot or GitHub App (e.g., dependabot, renovate, github-actions[bot]), the current behavior creates awkward situations:

- **Slack notifications** mention bots with `@bot-username`, which doesn't make sense
- **PR comments** are posted on bot PRs asking them to coordinate with humans
- **Issue reports** list bots as people who need to coordinate
- Humans are asked to "coordinate with a bot", which is unclear guidance

### Common bot scenarios

1. **Dependabot PR conflicts with human PR** - User needs to know their PR conflicts with a dependency update, but dependabot doesn't need to coordinate
2. **Two bot PRs conflict** (e.g., dependabot + renovate) - Neither bot can coordinate; humans should be aware but different handling needed
3. **GitHub Actions bot PRs** - Auto-generated PRs from workflows

## Suggested solutions

### Option 1: Filter out bot-authored PRs entirely
Add a configuration option to exclude PRs authored by bots:

```yaml
env:
EXCLUDE_BOT_PRS: "true" # Skip any PR authored by a bot
```

**Pros:**
- Simple to implement and understand
- Reduces noise from dependency update conflicts

**Cons:**
- Users might want to know when their PR conflicts with a bot PR
- Loses visibility into important conflicts

### Option 2: Asymmetric handling (recommended)
When a conflict involves one bot PR and one human PR:
- ✅ Notify the **human author** only (Slack, PR comment)
- ❌ Don't notify or comment on the **bot PR**
- ✅ Include conflict in reports/issues but note it's a bot PR

When both PRs are bots:
- ❌ Skip Slack notifications and PR comments
- ✅ Include in reports for visibility
- Note: "Both PRs are automated - review dependency/automation conflicts"

### Option 3: Configurable bot list
Allow users to specify which bots to filter:

```yaml
env:
EXCLUDE_BOT_AUTHORS: "dependabot[bot],renovate[bot],github-actions[bot]"
```

**Pros:**
- Flexible - users control which bots to exclude
- Can exclude some bots but not others

**Cons:**
- Requires users to know bot usernames
- Maintenance burden to keep list updated

### Option 4: Auto-detect using GitHub's bot flag
Use GitHub API's `user.type` field to detect bots automatically:

```python
if pr.user.type == 'Bot':
# Handle differently
```

**Pros:**
- No configuration needed
- Automatically handles all bots
- Future-proof for new bots

**Cons:**
- Relies on GitHub API correctly flagging bots

## Recommended implementation

**Combination of Option 2 + Option 4:**

1. Auto-detect bots using GitHub API's `user.type` field
2. Apply asymmetric handling:
- **Human vs Bot conflict**: Notify human only, skip bot notifications
- **Bot vs Bot conflict**: Skip all notifications, include in reports with note
3. Add optional override config:
```yaml
EXCLUDE_BOT_PRS: "false" # Default: include bot PRs but handle smartly
TREAT_AS_BOTS: "custom-automation-user" # Additional usernames to treat as bots
```

### Implementation details

1. Add bot detection in `pr_data.py`:
```python
@dataclass
class PRInfo:
number: int
title: str
author: str
is_bot: bool # New field
```

2. Update conflict categorization in `pr_conflict_detector.py`:
```python
# After deduplication, categorize conflicts
human_vs_human = [c for c in conflicts if not c.pr_a.is_bot and not c.pr_b.is_bot]
human_vs_bot = [c for c in conflicts if c.pr_a.is_bot ^ c.pr_b.is_bot]
bot_vs_bot = [c for c in conflicts if c.pr_a.is_bot and c.pr_b.is_bot]
```

3. Update notification modules:
- `slack_notify.py`: Only send for human_vs_human, include human-only mention for human_vs_bot
- `pr_comment.py`: Comment on human PR only for human_vs_bot conflicts
- `issue_writer.py`: Include all conflicts but annotate bot PRs

4. Add summary output:
```
Found 45 conflicts:
- 30 human vs human conflicts
- 12 human vs bot conflicts (notifying humans only)
- 3 bot vs bot conflicts (report only, no notifications)
```

## Additional considerations

- **Bot PR comment wording**: When commenting on a human PR about a bot conflict:
> ⚠️ **Potential merge conflict detected**
>
> This PR may conflict with automated PR [#123](link) by dependabot[bot].
>
> **Conflicting files:**
> - `package.json` (lines 10-15)
>
> Consider rebasing after the bot PR merges, or merge your PR first if it takes precedence.

- **Slack message wording** for human vs bot:
> 🤖 @alice - Your PR [#456](link) may conflict with automated PR [#123](link) by dependabot[bot]
> Files: `package.json`

## Testing checklist

- [ ] Detect bot using GitHub API `user.type` field
- [ ] Human vs Human: Full notifications (existing behavior)
- [ ] Human vs Bot: Notify human only
- [ ] Bot vs Bot: Report only, no notifications
- [ ] Bot detection works for: dependabot, renovate, github-actions[bot]
- [ ] TREAT_AS_BOTS config allows manual bot list
- [ ] README updated with bot handling behavior
- [ ] Deduplication still works correctly with bot flags

## Priority

**Medium** - This is a quality-of-life improvement that reduces notification noise and prevents awkward bot mentions, but doesn't break existing functionality.

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