Add organization member and repository discovery tools

### Describe the feature or problem you'd like to solve

There's no way to list organization members or enumerate repos within an org. Agents working in enterprise contexts can't answer basic questions like "who's on this team?" or "which repos does this org have?"

I hit this when building an onboarding assistant. My agent could create issues and PRs, but couldn't discover which repos existed in the org or who to assign as reviewers.

### Proposed solution

Add organization discovery operations:

| Tool | REST API Endpoint |
|------|-------------------|
| `list_org_members` | `GET /orgs/{org}/members` |
| `list_org_repos` | `GET /orgs/{org}/repos` |

Optionally also:
- `list_org_teams` → `GET /orgs/{org}/teams`
- `list_team_members` → `GET /orgs/{org}/teams/{team_slug}/members`

### Example prompts or workflows

1. "List all repositories in the acme-corp organization"
2. "Who are the members of this GitHub org?"
3. "Find all repos in the org that have open security alerts"
4. "Add all backend team members as reviewers on this PR"
5. "Which repos in the org haven't been updated in 6 months?"

### Additional context

Organization-level discovery is essential for enterprise workflows. The user's token permissions already gate access — this just exposes what the API already allows. Happy to submit a PR if this feature is accepted.