Feature: Add attachments support (linked PRs/commits)

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Issue co-written with dear assistant Claude Code 🤖

Feature Request

Add support for reading attachments on issues, particularly linked GitHub PRs and commits.

Current Behavior

When GitHub PRs/commits are linked to Linear issues (via magic words like fixes ENG-123), they appear in Linear's WebUI but are not exposed via linearis issues read.

linearis issues read ENG-123 | jq 'keys'
# Missing: attachments

Proposed Solution

Include attachments in issues read output:

{
  "identifier": "ENG-123",
  "attachments": [
    {
      "type": "GitHubPullRequest",
      "url": "https://github.com/org/repo/pull/42",
      "title": "Fix login bug",
      "state": "merged"
    },
    {
      "type": "GitHubCommit", 
      "url": "https://github.com/org/repo/commit/abc123",
      "message": "Fix login validation - fixes ENG-123"
    }
  ]
}

Optional Flag

Could be behind a flag to avoid extra API calls:

linearis issues read ENG-123 --with-attachments

Use Case

• Verify that PRs/commits are properly linked to issues
• Build automation that checks issue→PR linkage
• Generate reports of work done per issue