Add team key resolution for user list command

Support filtering users by team key (e.g., "ENG") in addition to team ID.

**Current behavior:**
`linear user list --team ENG` doesn't filter by team because team key to ID resolution is not implemented (lines 74-76 have a `pass` statement).

**Example usage:**
```bash
linear user list --team ENG
linear user list --team DESIGN
```

**Implementation:**
- Add team key to ID resolution logic (similar to issue.py resolve_team_id)
- Replace `pass` statement with actual resolution
- Call `client.get_teams()` to find team by key
- Pass resolved team_id to `client.get_users()`

**Files to modify:**
- `src/linear_cli/cli/commands/user.py` (list command, lines 74-76)