manager mcp: expose 'remind' tool sharing storage helper with agent surface

TODO.md

@@ -16,7 +16,7 @@
 - ~~Handle text overflow → suggest file_path option for long messages~~ ✓ fixed — Remind dispatch rejects `message.len() > 4096` (when no `file_path` was supplied) with an error pointing at the `file_path` escape hatch.
 - 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.
+- ~~**Manager-side `remind`**~~ ✓ fixed — `ManagerRequest::Remind` variant added, dispatch reuses `agent_server::store_remind` helper (shared across both surfaces), `mcp__hyperhive__remind` now on `ManagerServer` (auto-file lands at `/state/reminders/auto-<ts>.md` — manager's legacy state mount).
 - ~~**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.