feat(ai): turn natural-language requests into previewable OffPDF actions #78

Let a user describe a PDF task in plain language while keeping execution deterministic and reviewable.

Examples include: remove pages 2 and 4, rotate landscape pages, compress this file, or add page numbers.

Scope

• The model may select only actions already exposed by the typed OffPDF tool registry.
• Parse output into a strict schema; never execute arbitrary shell commands or model-generated paths.
• Show inputs, page ranges, options, and output names before running anything.
• Require explicit confirmation and use the existing job/cancellation flow.
• Reject ambiguous or unsupported requests instead of guessing.
• Keep prompt and document context local.

Acceptance criteria

• No action runs before the preview is confirmed.
• Invalid model output cannot reach a backend command.
• Plans are tested with deterministic fixtures, including malicious prompt content.
• The existing manual tools remain fully usable without AI.