Add first-class attachment download support (issue attachments + uploads links)

[dorkitude]

Problem

linctl can fetch issue metadata and expose attachment/file URLs, but there is no first-class way to download attached files directly via CLI.

In practice, this causes friction because:

• attachment URLs on uploads.linear.app are auth-gated
• users have to manually extract URLs from issue.description or attachments.nodes
• users must then manually apply auth headers/tokens to download files

This came up while retrieving a Linear issue attachment for END-6721 (double-encoded JSON ticket): the CLI returned issue details fine, but downloading the linked markdown required manual token/header handling.

Requested feature

Add built-in attachment download support to linctl.

Proposed UX

1. linctl issue get <ISSUE-ID> --download-attachments [--output-dir <dir>]
2. linctl issue attachment list <ISSUE-ID>
3. linctl issue attachment download <ISSUE-ID> [--all | --id <attachment-id> | --name <filename>] [--output <path>|--output-dir <dir>]

Behavior details

• Reuse existing authenticated session/token automatically.
• Support both:
  • canonical attachments entries
  • markdown file links in issue description/comments pointing to uploads.linear.app.
• Preserve server filename when possible.
• Exit non-zero on partial failures; print per-file status.
• --json output should include saved file paths and statuses.

Why this matters

• Removes manual token handling and reduces auth errors.
• Makes linctl more complete for agent workflows and terminal-only usage.
• Keeps sensitive download auth handling centralized and consistent.

Acceptance criteria

• From an authenticated linctl session, downloading an attachment from a known issue succeeds with one command.
• Works for both attachments.nodes and upload URLs embedded in markdown.
• Works in plaintext and JSON modes with clear status output.