Sort on-call pages by recent first by default

[platinummonkey]

What does this PR do?

Makes pup on-call pages list request newest on-call pages first by default by sending sort=-created_at to the unstable pages endpoint, and exposes --sort for created_at and -created_at.

Motivation

Teams with more than 1000 historical pages could not reach current pages because the command requested only oldest-first results from a capped endpoint. Defaulting to newest-first keeps current pages reachable while the stable paginated and time-filtered backend endpoint is unavailable.

Additional Notes

This keeps the existing unstable endpoint and does not add --from or --to; pagination remains limited by the backend.

Changes

• Added --sort to on-call pages list with default -created_at.
• Passed the sort query parameter to /api/unstable/on-call/pages.
• Added validation and tests for accepted and rejected sort values.
• Updated README.md and docs/COMMANDS.md summaries.

Testing

• cargo +stable fmt --check
• git diff --check
• Attempted cargo +stable test on_call_pages -- --test-threads=1, but cargo could not fetch the pinned datadog-api-client-rust git dependency because the sandbox allowlist returned HTTP 403.
• Attempted CARGO_NET_GIT_FETCH_WITH_CLI=true cargo +stable test on_call_pages -- --test-threads=1; it hit the same allowlist HTTP 403.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

Closes #709

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Bits Code status: ✅ Done

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