Encode per-endpoint pagination bounds in the operation declarations

[saramaebee]

Follow-up to #38.

PageArgs::resolve applies one global pagination policy (defaults, clamp to 1..=100), but several endpoints have their own bounds the declarations don't express:

• GET /v2/issues clamps count to a minimum of 5
• GET /revisions/{locator}/snippets caps pageSize at 50 (currently enforced ad hoc in the Snippet list_page impl)

Idea: encode per-entity bounds in the param structs / declarations themselves so that

1. both surfaces enforce the same bounds,
2. the MCP input schemas advertise them (agents see constraints instead of discovering them via API errors), and
3. --help makes the CLI act as documentation for what each endpoint supports.

Not needed for #38 — deliberately deferred.