feat: add top-level command aliases (comment, assign, tag, note, priority)

[DreadPirateRobertz]

Summary

• Adds intuitive top-level command shortcuts that delegate to existing functionality:
  • bd comment <id> "text" → bd comments add <id> "text" (91 failed attempts before discovery)
  • bd assign <id> <name> → bd update <id> --assignee <name> (~20 attempts)
  • bd tag <id> <label> → bd update <id> --add-label <label> (~15 attempts)
  • bd note <id> "text" → bd update <id> --notes "text" (~10 attempts)
  • bd priority <id> <n> → bd update <id> --priority <n> (~10 attempts)
• All aliases include tab-completion support via issueIDCompletion
• No existing commands or behavior is changed

Test plan

• bd comment <id> "test comment" adds a comment
• bd assign <id> username sets assignee
• bd tag <id> bug adds a label
• bd note <id> "some notes" sets notes
• bd priority <id> 1 sets priority to P1
• All existing commands still work unchanged

Addresses #2611

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