feat(discord): add /remind slash command

[chaodu-agent]

Summary

Adds a /remind slash command that lets humans schedule one-shot delayed mentions to users/roles in the channel.

Closes #796

What Changed

• src/remind.rs — New module: Reminder struct, ReminderStore (JSON persistence), delay parser, scheduler
• src/discord.rs — Register /remind command with 3 string options (targets, message, delay); handler validates input, persists, and schedules via tokio; re-schedules pending reminders on ready
• src/main.rs — Wire up ReminderStore and pass to Discord Handler
• docs/slash-commands.md — Document the new command

Usage

/remind targets:@超渡法師 @普渡法師 message:Review PR #42 delay:2h

Fires after 2 hours:

⏰ Reminder from @pahud:
"Review PR #42"
cc @超渡法師 @普渡法師

Design Decisions

• String option for targets — Discord Mentionable type only allows one mention per option; using a string lets users specify multiple @user @role mentions
• JSON file persistence — Simple, no external deps; stored at $HOME/.openab/reminders.json
• tokio::time::sleep — Lightweight in-process scheduling; no need for external scheduler for one-shots
• Human-only — cmd.user.bot check prevents bot-to-bot reminder loops

Testing

• Unit tests for delay parser (parse_delay) and formatter (format_delay)
• Manual testing requires a Discord bot token (not available in CI)

Not Included (future work)

• Recurring reminders (see feat: usercron — hot-reload cronjob.toml for agent-managed schedules #628 for usercron)
• /remind list / /remind cancel subcommands
• Multi-platform support (Slack, gateway)

[github-actions]

⚠️ This PR is missing a Discord Discussion URL in the body.

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Please edit the PR description to include a link like:

Discord Discussion URL: https://discord.com/channels/...

This PR will be automatically closed in 3 days if the link is not added.

[chaodu-agent]

LGTM ✅ — All findings addressed, CI green.

What This PR Does

Adds a /remind slash command that lets humans schedule one-shot delayed mentions to users/roles in the channel. Reminders persist to JSON and survive bot restarts.

How It Works

src/remind.rs provides ReminderStore (Arc<Mutex> + JSON file) and schedule_reminder (tokio::spawn + sleep). src/discord.rs registers the /remind command and handles interactions; the ready event re-schedules persisted reminders on startup.

Findings

#	Severity	Finding	Status
1	🟢	Broken unit test fixed — correct assertions now	✅ Fixed
2	🟢	Mutex released before file I/O — no longer blocks executor	✅ Fixed
3	🟢	Serialization error logged and write skipped — no silent data loss	✅ Fixed
4	🟢	Per-user rate limit (max 5 active reminders) added	✅ Fixed
5	🟢	Message length cap (1800 chars) enforced	✅ Fixed
6	🟢	@everyone/@here neutralized with zero-width space	✅ Fixed
7	🟢	create_dir_all ensures persistence directory exists	✅ Fixed
8	🟢	Dedup via scheduled_ids prevents double-fire on reconnect	✅ Fixed
9	🟢	Reminder only removed after successful send	✅ Fixed

What's Good (🟢)

• All original review findings addressed in two focused commits
• CI fully green (cargo check + clippy + test + 7 Docker smoke-tests)
• Clean module separation maintained
• Good unit test coverage for delay parsing

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Reviewers: 普渡法師 ✅, 擺渡法師 ✅, 覺渡法師 ✅ | Aggregated by 超渡法師