feat: add cron tool integration with agent

- Add adhocore/gronx dependency for cron expression parsing
- Fix CronService race conditions and add cron expression support
- Add CronTool with add/list/remove/enable/disable actions
- Add ContextualTool interface for tools needing channel/chatID context
- Add ProcessDirectWithChannel to AgentLoop for cron job execution
- Register CronTool in gateway and wire up onJob handler
- Fix slice bounds panic in addJob for short messages

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

go.sum

@@ -1,4 +1,6 @@
 cloud.google.com/go/compute/metadata v0.3.0/go.mod h1:zFmK7XCadkQkj6TtorcaGlCW1hT1fIilQDwofLpJ20k=
+github.com/adhocore/gronx v1.19.6 h1:5KNVcoR9ACgL9HhEqCm5QXsab/gI4QDIybTAWcXDKDc=
+github.com/adhocore/gronx v1.19.6/go.mod h1:7oUY1WAU8rEJWmAxXR2DN0JaO4gi9khSgKjiRypqteg=
 github.com/bwmarrin/discordgo v0.29.0 h1:FmWeXFaKUwrcL3Cx65c20bTRW+vOb6k8AnaP+EgjDno=
 github.com/bwmarrin/discordgo v0.29.0/go.mod h1:NJZpH+1AfhIcyQsPeuBKsUtYrRnjkyu0kIVMCHkZtRY=
 github.com/caarlos0/env/v11 v11.3.1 h1:cArPWC15hWmEt+gWk7YBi7lEXTXCvpaSdCiZE2X5mCA=