Add first-class issue attachment download support

[dorkitude]

Summary

Adds first-class issue attachment discovery and download support for #47.

Implemented user-facing flows:

• linctl issue attachment list <issue-id> lists canonical Linear attachments plus uploads.linear.app links found in issue descriptions and recent comments.
• linctl issue attachment download <issue-id> downloads a single attachment automatically when there is only one entry.
• linctl issue attachment download <issue-id> --all --output-dir <dir> downloads all downloadable entries.
• linctl issue attachment download <issue-id> --id <attachment-id> selects a canonical attachment by ID.
• linctl issue attachment download <issue-id> --name <filename> --output <path> selects by title or URL filename and writes to a specific path.
• linctl issue get <issue-id> --download-attachments --output-dir <dir> fetches issue details and downloads attachments in one command.

Behavior notes:

• Download output works in table, plaintext, and JSON modes.
• Partial download failures are reported per file and cause a non-zero exit.
• Non-downloadable GitHub issue/PR links are skipped during --all instead of failing the whole operation.
• Filenames are resolved from Content-Disposition, attachment title, or URL path and sanitized before writing.
• Existing files are not overwritten for directory downloads; names get a numeric suffix.
• The Linear auth header is only sent to uploads.linear.app downloads, not arbitrary external attachment URLs.

Docs and tests:

• Updates README, SKILL.md, and master_api_ref.md for the new commands and behavior.
• Adds focused command tests in cmd/issue_cmd_test.go for upload URL extraction, entry collection, selector behavior, skipped non-downloadable links, failure detection, flag registration, and auth-header handling for external downloads.

Closes #47

Testing

• go test ./...
• go run . issue attachment --help
• go run . issue get --help

Not Tested

• Live download against a real authenticated uploads.linear.app attachment. The code path is covered with unit tests, but this environment does not have a live Linear attachment fixture.