feat(core): add cursor persistence module for session-based timestamps

Introduces a lightweight file-based cursor system for persisting
per-user timestamps across CLI invocations. This enables "since last
check" semantics where `lore me` can track what the user has seen.

Key design decisions:
- Per-user cursor files: ~/.local/share/lore/me_cursor_<username>.json
- Atomic writes via temp-file + rename pattern (crash-safe)
- Graceful degradation: missing/corrupt files return None
- Username sanitization: non-safe chars replaced with underscore

The cursor module provides three operations:
- read_cursor(username) -> Option<i64>: read last-check timestamp
- write_cursor(username, timestamp_ms): atomically persist timestamp  
- reset_cursor(username): delete cursor file (no-op if missing)

Tests cover: missing file, roundtrip, per-user isolation, reset
isolation, JSON validity after overwrites, corrupt file handling.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/mod.rs

@@ -2,6 +2,7 @@ pub mod backoff;
 pub mod config;
 #[cfg(unix)]
 pub mod cron;
+pub mod cursor;
 pub mod db;
 pub mod dependent_queue;
 pub mod error;