feat(cron): add lore cron command for automated sync scheduling

Add lore cron {install,uninstall,status} to manage a crontab entry that
runs lore sync on a configurable interval. Supports both human and robot
output modes.

Core implementation (src/core/cron.rs):
  - install_cron: appends a tagged crontab entry, detects existing entries
  - uninstall_cron: removes the tagged entry
  - cron_status: reads crontab + checks last-sync time from the database
  - Unix-only (#[cfg(unix)]) — compiles out on Windows

CLI wiring:
  - CronAction enum and CronArgs in cli/mod.rs with after_help examples
  - Robot JSON envelope with RobotMeta timing for all 3 sub-actions
  - Dispatch in main.rs

Also in this commit:
  - Add after_help example blocks to Status, Auth, Doctor, Init, Migrate,
    Health commands for better discoverability
  - Add LORE_ICONS env var documentation to CLI help text
  - Simplify notes format dispatch in main.rs (removed csv/jsonl paths)
  - Update commands/mod.rs re-exports for cron + notes cleanup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/mod.rs

@@ -1,5 +1,7 @@
 pub mod backoff;
 pub mod config;
+#[cfg(unix)]
+pub mod cron;
 pub mod db;
 pub mod dependent_queue;
 pub mod error;