feat: add milestone support to issue commands

[jholm117]

Summary

• Adds --milestone flag to issue create and issue update commands
• Resolves milestones by name (case-insensitive) within the specified project
• For issue update, automatically falls back to the issue's existing project if --project is not explicitly provided
• Validates that a project context exists before attempting milestone lookup, with helpful error messages
• Shows project and milestone in issue view output (displayed below the title when present)

Example usage

# Create an issue with a milestone
linear issue create --title "Implement feature" --team ENG --project "My Project" --milestone "Phase 1"

# Update an issue's milestone (project inferred from issue)
linear issue update ENG-123 --milestone "Phase 2"

# View an issue — now shows project and milestone
linear issue view ENG-123
# => Project: My Project | Milestone: Phase 1

Test plan

• Happy path tests for create and update with milestone
• Snapshot test for issue view with project and milestone displayed
• All 205 tests pass
• deno check and deno lint pass
• GraphQL codegen regenerated
• Verified end-to-end against Linear API

🤖 Generated with Claude Code

[jholm117]

I am a human and I verify this works.

[schpet]

@jholm117 haha appreciate that 😄 – i didn't test it out but trust your endorsement. if you notice anything please throw up a follow up PR before a i cut a release (not sure when that is, yet)