Add plugins/all package to simplify built-in plugin registration for embedded engines

[Copilot]

Embedding the workflow engine required importing and wiring 25+ plugin packages individually. This adds a plugins/all package as a single entry point for all built-in plugins.

New: plugins/all

• DefaultPlugins() []plugin.EnginePlugin — returns a fresh slice of all standard plugins; safe to modify before loading (add, remove, reorder)
• LoadAll(engine PluginLoader) error — loads all default plugins in one call; PluginLoader is a minimal interface satisfied by *workflow.StdEngine

engine := workflow.NewStdEngine(app, logger)
if err := allplugins.LoadAll(engine); err != nil {
    log.Fatalf("failed to load plugins: %v", err)
}

// Or customize the set:
plugins := allplugins.DefaultPlugins()
plugins = append(plugins, myCustomPlugin)
for _, p := range plugins {
    engine.LoadPlugin(p)
}

Updated callers

• cmd/server/main.go — defaultEnginePlugins() now delegates to allplugins.DefaultPlugins(), replacing 25 individual plugin imports
• example/main.go — migrated from the legacy handlers.New*WorkflowHandler() pattern to allplugins.LoadAll(engine)

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[Copilot]

Pull request overview

This PR adds a plugins/all package that provides a single entry point for registering all 25 built-in workflow engine plugins, eliminating the need to manually import and wire each plugin package individually. This significantly simplifies the setup for applications embedding the workflow engine.

Changes:

• Added plugins/all package with DefaultPlugins() and LoadAll() functions
• Updated cmd/server/main.go to use allplugins.DefaultPlugins() instead of manually listing 25 plugin imports
• Migrated example/main.go from individual handler registration to allplugins.LoadAll(engine) pattern

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
plugins/all/all.go	New package providing DefaultPlugins() and LoadAll() convenience functions for plugin registration
plugins/all/all_test.go	Comprehensive test suite covering plugin uniqueness, independence, loading, and error handling
cmd/server/main.go	Replaced 25 individual plugin imports with single allplugins.DefaultPlugins() call
example/main.go	Migrated from legacy handler registration to plugin-based loading via allplugins.LoadAll()
example/go.mod	Added transitive dependencies required by newly-loaded plugins (cache, reverseproxy, scheduler, gobwas/glob, robfig/cron, go-chi/chi)
example/go.sum	Updated checksums for new transitive dependencies