feat: add agent support with delegation, labels, and mentions

[StuartFenton]

Summary

Adds support for delegating issues to Linear agents, managing agent sessions, and @mentioning agents.

Changes

• Add --delegate flag to issue create and issue update for agent assignment
• Add --label flag to issue create for setting labels (repeatable, case-insensitive)
• Add linctl agent <ID> command to view agent session status and activities
• Add linctl agent mention <ID> <message> to @mention agents
• Add GetTeamLabels(), GetOrganization(), MentionAgent(), DeleteComment() API functions
• Add comprehensive tests for new functionality

Usage

# Create issue and delegate to agent
linctl issue create --title "Task" --team ENG --delegate "Agent Name"

# Create with labels
linctl issue create --title "Task" --team ENG --label mylabel

# Check agent session status
linctl agent ENG-123
linctl agent ENG-123 --json

# @mention an agent to trigger them
linctl agent mention ENG-123 "Please fix the authentication bug"

Testing

All tests pass: go test ./...