reminder: file_path delivery + extract scheduler into own module

TODO.md

@@ -15,7 +15,7 @@
 - Per-agent reminder limits (burst capacity, rate limiting)
 - ~~**Expose `remind` MCP tool**~~ ✓ fixed — `mcp__hyperhive__remind` now on `AgentServer`; takes `message`, exactly one of `delay_seconds` / `at_unix_timestamp`, optional `file_path`. Manager surface still missing (no `ManagerRequest::Remind` variant) — separate item below.
 - **Manager-side `remind`**: mirror of the agent tool but on `ManagerServer`. Needs `ManagerRequest::Remind` variant in hive-sh4re, dispatch in manager_server.rs, MCP tool wiring.
-- **File path delivery**: currently unused in scheduler delivery loop — implement file write/delivery to /state/<agent>/reminders/ or similar (also needed for the overflow-check escape hatch above to actually do anything useful).
+- ~~**File path delivery**~~ ✓ fixed — scheduler now writes the reminder body to the requested `file_path` (mapped from container `/agents/<agent>/state/...` to host `/var/lib/hyperhive/agents/<agent>/state/...`) and delivers a short pointer message in its place. Path-traversal + foreign-agent-state writes are rejected; on rejection or write failure the body falls back to inline delivery with a noted warning. New module `hive-c0re/src/reminder_scheduler.rs` (extracted from main.rs).
 - ~~**Orphan reminders**~~ ✓ fixed — `Broker::deliver_reminder` wraps the inbox INSERT + reminders UPDATE in one sqlite transaction; partial failure can no longer cause duplicate delivery on the next tick.
 - ~~**Unbounded batches**~~ ✓ fixed — scheduler now calls `get_due_reminders(REMINDER_BATCH_LIMIT)` (cap = 100/tick); overflow stays due and gets picked up next cycle.
 - **Scheduler shutdown**: add graceful shutdown signal when coordinator is destroyed (currently runs forever)