Add MCP transcript pagination

[pranc1ngpegasus]

Summary

• add loopback_seconds to ask_current_conversation, defaulting to 600 seconds
• add tool-call pagination with cursor and limit
• return continuation data in the existing text context and namespaced _meta
• keep pagination state stable across live appends by binding cursors to the session, view, time window, and original append boundary
• document the behavior and isolate the pagination implementation in its own module

Why

MCP questions previously received every visible transcript segment with no way to bound the time window or retrieve a long transcript incrementally. This adds a bounded default context and optional pagination while preserving the existing local IPC endpoint and MCP 2025-03-26 text response compatibility.

Behavior

• the lookback window is measured from the latest non-empty segment's end time
• omitting limit returns all non-empty segments overlapping the window
• setting limit returns the last entries in Wisp display order and exposes next_cursor for the preceding page
• continuation calls provide both cursor and limit
• the desktop /conversation IPC response remains unchanged

Validation

• cargo fmt --all -- --check
• cargo test --workspace --exclude wisp-desktop --all-targets
• cargo clippy --workspace --exclude wisp-desktop --all-targets -- -D warnings
• git diff --check