feat: add parent and sub-issues to issue view output

[paymog]

Summary

• Add parent issue display to linear issue view output (when the issue has a parent)
• Add sub-issues list to linear issue view output (when the issue has children)
• Enables agents and users to easily navigate up and down the issue hierarchy

Closes #84

Example Output

# Implement user authentication

Add user authentication to the application.

Parent:
  TEST-100: Epic: Security Improvements [In Progress]

Sub-issues:
  TEST-457: Add login form [Done]
  TEST-458: Add password reset flow [Todo]
  TEST-459: Add OAuth support [In Progress]

Test plan

• Added new test case "With Parent And Sub-issues" to verify hierarchy display
• Updated existing tests to include parent/children fields in mock data
• All snapshot tests updated and passing
• deno check passes
• deno lint passes

🤖 Generated with Claude Code

[schpet]

nice thanks! this lgtm but can you make the tty and non-tty output consistent? i.e. Sub-issues should always be an h2 (## prefix in markdown), have a newline underlneath it, etc.

desired output:

## Sub-issues

- **CLI-65**: Handle JPG images in descriptions _[Backlog]_
- **CLI-64**: Handle PNG images in descriptions _[Backlog]_

[paymog]

this is now fixed @schpet

[schpet]

@paymog thank you!

[schpet]

this will land in 1.6.0 shortly