fix(cli): fail closed on destructive confirmations [EXT-69]

[ruby-automation]

Summary

• Add Prompt.confirm_destructive?/1 with a fail-closed default: false.
• Use it for filter-only issue unassign and both issue move confirmation paths.
• Preserve the affirmative Prompt.yes?/1 behavior for non-destructive prompts.
• Add EOF, blank, invalid-input, explicit-answer, and command-level mutation guards.
• Record the superseding Phase 12 confirmation decision and update the command documentation.

Verification

• mise exec -- mix ci — passed (606 app tests, 58 root tests, Credo clean, audits clean).
• Focused prompt, unassign, and move tests — passed (126 tests).
• git diff HEAD^ HEAD --check — passed.

Linear issue: https://linear.app/the-rubyists/issue/EXT-69/make-bulk-mutation-confirmation-fail-safe-on-eof-and-non-tty-input