Make hey box --all fetch every page

[code-monger-givenall]

Follow-up to #55.

hey box <box> --all still stops after 100 history pages. In Imbox, that means 30 initial items plus 100 batches of 10, so larger mailboxes stop at 1,030 even when next_history_url is still present.

This change follows the cursor until HEY returns no next page. It also:

• Detects a repeated cursor and returns a clear error instead of looping.
• Continues through an empty intermediate page when that page supplies another cursor.
• Keeps the existing same-origin check on every pagination URL.
• Removes the obsolete notice that described a deliberate 100-page cutoff.

Tests

• env GOWORK=off mise x go@1.26.6 golangci-lint@2.12.2 -- make check
• Unit coverage fetches 101 additional pages and returns all 1,040 items.
• Unit coverage rejects a repeated cursor and continues through an empty intermediate page.
• Earlier live read-only validation exhausted a mailbox cursor chain past the previous 1,030-item ceiling.

No mailbox content is changed by this command.

[Copilot]

Pull request overview

Updates box pagination so hey box <box> --all follows every cursor safely.

Changes:

• Removes the 100-page limit.
• Detects repeated cursors and continues through empty intermediate pages.
• Adds regression coverage and removes the obsolete truncation notice.

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Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/cmd/box.go	Implements uncapped, loop-safe pagination.
internal/cmd/box_test.go	Tests long pagination, cursor loops, and empty pages.

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