feat: auto-reload config file on changes

[hyldmo]

Summary

• Watches the config file (numux.config.ts/.js) for changes and automatically applies updates to running processes — restarting modified, starting added, stopping removed
• Enabled by default in TUI mode when using a config file; opt out with --no-config-watch
• Skipped automatically for prefix mode, CLI-command mode, and all-one-shot configs

Changes

• src/config/diff.ts — Pure diffConfigs() function comparing two configs (added/removed/modified)
• src/config/config-watcher.ts — watchConfig() using fs.watch() with 500ms debounce
• src/config/loader.ts — Added resolveConfigPath() and cache-busting on import()
• src/process/manager.ts — applyConfigChange() to stop/restart/start processes based on diff
• src/ui/app.ts — Dynamic pane add/remove, config watcher lifecycle
• src/ui/tabs.ts — addProcess()/removeProcess() for dynamic tab management
• src/cli-flags.ts / src/cli.ts — --no-config-watch flag
• src/types.ts — added/removed event types

Test plan

• bun test — 413/413 pass
• bun run typecheck — clean
• bun run lint — clean
• Manual: run with config file, edit to add/remove/modify a process, verify live update
• Manual: --no-config-watch disables the watcher
• Manual: invalid config edit logs error, keeps running with old config

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