feat: add comprehensive AI coding instructions for Thoughtful Discord Bot

.github/copilot-instructions.md

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+# Thoughtful Discord Bot - AI Coding Instructions
+
+## Project Overview
+A Go Discord bot that integrates with the Thoughtful API for managing ideas through Discord commands. Built with `discordgo` library, uses JSON for user config persistence.
+
+## Architecture
+
+### Component Structure
+- **bot.go**: Main entry point. Handles Discord session, registers handlers, manages bot lifecycle
+- **commands/**: Modular command system where each command is a separate file
+  - `commands.go`: Command registry and interaction router
+  - `storage.go`: JSON-based user config persistence (users.json)
+  - Individual command files: `setup.go`, `list.go`, `delete.go`, `logout.go`
+
+### Key Design Patterns
+
+**Command Registration Pattern**: Each command implements the `Cmd` struct:
+```go
+type Cmd struct {
+    Command             *discordgo.ApplicationCommand
+    Handler             func(s *discordgo.Session, ic *discordgo.InteractionCreate)
+    AutocompleteHandler func(s *discordgo.Session, ic *discordgo.InteractionCreate) // optional
+}
+```
+All commands are registered via `getAllCommands()` in [commands/commands.go](../commands/commands.go).
+
+**Dual Command Interface**: The bot supports both:
+- Slash commands (e.g., `/setup`, `/list`) via Discord's ApplicationCommand API
+- Legacy text commands (e.g., `.thought Title; Description`) for backward compatibility
+
+**User State Management**: Per-user configs (Thoughtful instance URL + API key) stored in `users.json`. Thread-safe access via mutex in [commands/storage.go](../commands/storage.go).
+
+**Modal-Based Setup**: `/setup` uses Discord modals (not option prompts) to collect sensitive credentials privately.
+
+### External Dependencies
+- **Thoughtful API**: External idea management service. Requires:
+  - Base URL (user-provided, e.g., `https://instance.com`)
+  - API key authentication via `API-Authentication` header
+  - Endpoints: `/api/ideas/create`, `/api/ideas/list`, `/api/ideas/delete`
+- **Discord API**: via `github.com/bwmarrin/discordgo` library
+
+## Developer Workflows
+
+### Running Locally
+```sh
+# Set environment variable
+$env:DISCORD_BOT_TOKEN="your_token_here"
+
+# Run directly
+go run .
+```
+
+### Docker Deployment
+The bot is designed for Docker Compose deployment. See [docker-compose.yml](../docker-compose.yml):
+- Uses `golang:1.25` image (note: uncommon version - likely should be `1.21` or `1.22`)
+- Auto-pulls git changes on container start
+- Requires `.env` file with `DISCORD_BOT_TOKEN`
+
+### Adding New Commands
+1. Create new file in `commands/` (e.g., `mycommand.go`)
+2. Implement `newMyCommand()` returning `*Cmd`
+3. Add to `getAllCommands()` in [commands/commands.go](../commands/commands.go)
+4. Command automatically registers on bot startup
+
+## Project-Specific Conventions
+
+### Error Handling
+- Use `respondError()` helper for interaction failures (defined in command files)
+- Silent failures acceptable for autocomplete handlers
+- Channel messages for `.thought` command errors
+
+### User Identification
+Discord interactions may have `User` OR `Member.User` depending on context:
+```go
+user := ic.User
+if user == nil {
+    user = ic.Member.User
+}
+```
+Always check both when accessing user data.
+
+### API Communication
+- Always trim trailing slashes from instance URLs: `url = strings.TrimSuffix(url, "/")`
+- Use `API-Authentication` header (not `Authorization`)
+- 401 status codes mean invalid/expired API keys
+
+### Storage Layer
+- **Never** edit `users.json` directly - always use `storage.go` functions
+- Operations are mutex-protected but not transactional
+- Corrupted JSON automatically resets to empty map
+
+## Critical Gotchas
+
+1. **Intents Required**: Bot needs `IntentsGuildMessages` (set in [bot.go](../bot.go#L40)). Message Content Intent must be enabled in Discord Developer Portal.
+
+2. **Slash Command Updates**: Slash commands persist in Discord. To update, uncomment command cleanup code in [commands/commands.go](../commands/commands.go#L30-L33).
+
+3. **Modal Response Type**: Setup uses `InteractionResponseModal`, not `InteractionResponseChannelMessageWithSource`.
+
+4. **Autocomplete Filtering**: Delete command autocomplete should filter by user input but currently returns all ideas - see [commands/delete.go](../commands/delete.go#L86-L100).
+
+5. **No Database**: All state in `users.json`. Loss of this file = all users must re-run `/setup`.