Add team key resolution for user list command

[AdiKsOnDev]

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:

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)