Add cursor-based pagination to listSessions

[connor4312]

Summary

Adds optional cursor + limit pagination to the root listSessions command so a client with a large session catalogue can load it incrementally instead of receiving every SessionSummary in a single response. Modeled on MCP's opaque-cursor pagination, plus an optional limit the server SHOULD respect (but MAY cap).

Fully additive: a caller that omits the new fields, and a host that does not paginate, behave exactly as today.

Closes #275.

Shape

The pagination inputs/outputs are factored into reusable base interfaces in types/common/commands.ts, so future paginated commands can just extend them and the pagination contract lives in one place:

export interface PaginatedParams {
  limit?: number;   // server SHOULD respect; MAY cap
  cursor?: string;  // opaque; omit for the first page
}

export interface PaginatedResult {
  nextCursor?: string; // absent ⇒ no more pages
}

export interface ListSessionsParams extends BaseParams, PaginatedParams {
  channel: 'ahp-root://';
  filter?: object;
}

export interface ListSessionsResult extends PaginatedResult {
  items: SessionSummary[];
}

• The cursor is opaque and server-defined — ordering and keyset are the host's choice, mirroring how fetchTurns exposes before/hasMore without dictating storage. Clients MUST NOT parse, modify, or persist it; an unrecognised cursor SHOULD be rejected with InvalidParams.
• The server SHOULD return most-recently-modified first, so the first page is the immediately useful one.
• The root/session* notifications keep an already-fetched page live exactly as today; pagination governs only the initial and backfill fetches.

Changes

• types/ — new PaginatedParams / PaginatedResult; ListSessionsParams / ListSessionsResult extend them; both exported from types/index.ts.
• Generators — added the two base interfaces to the knownSpecial exhaustiveness lists in the rust/go/kotlin/swift generators (like BaseParams, they're flattened via extends, not emitted standalone).
• Regenerated — JSON Schema and all five client mirrors (Rust, Kotlin, Swift, TypeScript, Go); the native clients flatten limit/cursor/nextCursor into ListSessions*.
• Docs — a "Paginating the catalogue" section in docs/specification/root-channel.md.
• CHANGELOGs — an entry in the spec changelog plus every client changelog (a types/** change ripples to all clients).

Open questions from the issue (deliberately out of scope)

• Sorting/filtering (raised by @roblourens): kept orthogonal — filter stays an untyped object as today. Pagination is additive so a typed filter/sort can land later without reworking this.
• approximateTotal: omitted for v1.

Validation

• npm run test → 275 pass, 100% reducer branch coverage, changelog + release-metadata gates green; npm run lint clean.
• npm run generate produces no generator warnings.
• cargo build, go build ./..., and the TypeScript client tsc --noEmit all pass.

[roblourens]

How is filter interpreted? Should we add a sort criteria?

[connor4312]

Yea, idk, I forgot this was a thing. I'll just remove it for now