feat(acp): paginate session list

[kalvinnchau]

summary

implements acp pagination for session/list, per https://agentclientprotocol.com/protocol/session-list#pagination

• uses a fixed backend page size of 50
• returns nextCursor when more sessions are available
• accepts cursor from the previous response
• honors cwd as an exact absolute-path filter
• rejects malformed cursors, relative cwd, and cursor/cwd mismatches with invalid_params

notes

this is a behavioral change for acp clients: session/list {} now returns the first page instead of every visible session.
clients must follow nextCursor to retrieve all sessions.

the cursor stores both updated_at and session_id because goose timestamps can collide at second precision. this avoids
skipping or duplicating sessions when multiple rows share the same timestamp.

[DOsinga]

thanks -- this PR is the right idea but I think we need to clean up a few things here before we merge:

list_nonempty_sessions_by_types_paged re-implements the column list, JOIN, GROUP BY, and bind logic of list_sessions_by_types. Refactor to one query function with optional filters/cursor/limit, otherwise this is going to be a continuous maintenance burden.

insert_session_for_list looks like a method that is only there for testing, but that in effect means we are testing what that method does, not what happens in the wild.

moving session/list from ? to respond_with_error is a behavior change for existing clients. Mention it in the PR description and cc @alexhancock to see if this breaks the existing TUI library.

I would also hash the filters into the cursor to make this more future proof when we want to add more to the filter -- like the session type.