Add on-call pages list command

[nadimchakra]

Summary

Adds on-call pages list, filling the gap where pup could only create or get <page_id> an on-call page — there was no way to list pages for a team or responder. Filters by team handle server-side (filter=team:<handle>) and by responder user id client-side.

Notable Changes

• src/commands/on_call.rs: pages_list(cfg, team, responder, page_size) via client::raw_get (same pattern as pages_get), plus a page_has_responder helper for the client-side responder filter.
• src/main.rs: List variant on OnCallPagesActions with --team, --responder, --page-size; wired into routing. Verb-based classification makes it read-only.
• docs/COMMANDS.md: on-call row updated to pages (list, get, create).
• Tests: happy path, error path, and responder-match/reject/missing-field cases.

Implementation notes

• Uses the unstable/on-call/pages endpoint deliberately: datadog-api-client exposes no list binding, and the stable GET /api/v2/on-call/pages returns an empty 200 (content-length: 0) even with a filter. See the companion issue for the API-side gap.
• --responder is applied client-side because the endpoint has no responder facet (responders:, responder:, user:, target: all return zero results).
• on_call_read is already in default_scopes() / read_only_scopes(), so no auth change is needed.

Known limitations (inherited from the endpoint)

• No server-side sort, no working pagination (page[number]/page[offset] ignored — always oldest-first, page 1), no in-query time filter. page[size] caps at 1000, so teams with >1000 total pages can't retrieve recent ones via this command. These are API-side and tracked in the companion issue.

Draft: opened to let CI validate the build — I couldn't compile locally (corporate crates.io mirror was missing russh 0.61.1).

🤖 Generated with Claude Code

[nadimchakra]

Companion issue documenting the API-side gap and the unstable/on-call/pages endpoint limitations: #642