Add GitHub tool to chat handler

[oldsj]

Problem

Chat agent said "I don't have access to GitHub" when user asked about open issues. User had to correct it.

Solution

Add github tool to the chat handler using existing github_pr.py service:

• Read access: list_issues, list_prs, get_repo, get_issue
• Issue management: create_issue, update_issue, add_comment
• NO PR creation (worker agents only via spawn_task)

Files to Modify

1. backend/src/mainloop/services/github_pr.py - Add list_open_issues()
2. backend/src/mainloop/services/chat_handler.py - Add github tool

Implementation

1. Add list_open_issues() to github_pr.py

class IssueSummary(BaseModel):
    """Summary of a GitHub issue."""
    number: int
    title: str
    state: str
    author: str
    created_at: datetime
    updated_at: datetime
    url: str
    labels: list[str] = []


async def list_open_issues(repo_url: str, limit: int = 10) -> list[IssueSummary]:
    """List open issues for a repository."""
    owner, repo = _parse_repo(repo_url)
    gh = _get_github()

    try:
        response = await gh.rest.issues.async_list_for_repo(
            owner=owner, repo=repo, state="open", per_page=limit,
        )
        # Filter out PRs (GitHub API returns PRs as issues too)
        return [
            IssueSummary(
                number=issue.number,
                title=issue.title,
                state=issue.state,
                author=issue.user.login,
                created_at=issue.created_at,
                updated_at=issue.updated_at,
                url=issue.html_url,
                labels=[label.name for label in issue.labels] if issue.labels else [],
            )
            for issue in response.parsed_data
            if not hasattr(issue, 'pull_request') or issue.pull_request is None
        ]
    except Exception:
        return []

2. Add github tool to chat_handler.py

Create tool with actions: list_issues, list_prs, get_repo, get_issue, create_issue, update_issue, add_comment

3. Update system prompt

base_prompt = """You are a helpful AI assistant that can also spawn autonomous coding agents.

You have access to:
- github: Read repo info and manage issues (list_issues, list_prs, get_repo, get_issue, create_issue, update_issue, add_comment)
- spawn_task: Spawn autonomous coding agents for development work

For information/issue management, use github.
For coding tasks (modify code, create PRs, run tests), use spawn_task after confirming with the user."""

Permissions Model

Tool	Capability
github	Read: list_issues, list_prs, get_repo, get_issue
github	Write: create_issue, update_issue, add_comment
spawn_task	Spawns worker agent (has full access including PR creation)