refactor: extract AppEvent to crates/ironclaw_common

SseEvent was defined in src/channels/web/types.rs but imported by 12+
modules across agent, orchestrator, worker, tools, and extensions — it
had become the application-wide event protocol, not a web transport
concern.

Create crates/ironclaw_common as a shared workspace crate and move the
enum there as AppEvent.  Also move the truncate_preview utility which
was similarly leaked from the web gateway into agent modules.

- New crate: crates/ironclaw_common (AppEvent, truncate_preview)
- Rename SseEvent → AppEvent, from_sse_event → from_app_event
- web/types.rs re-exports AppEvent for internal gateway use
- web/util.rs re-exports truncate_preview
- Wire format unchanged (serde renames are on variants, not the enum)

Aligned with the event bus direction on refactor/architectural-hardening
where DomainEvent (≡ AppEvent) is wrapped in a SystemEvent envelope.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/extensions/manager.rs

@@ -1131,7 +1131,7 @@ impl ExtensionManager {
     /// Broadcast an extension status change to the web UI via SSE.
     async fn broadcast_extension_status(&self, name: &str, status: &str, message: Option<&str>) {
         if let Some(ref sse) = *self.sse_manager.read().await {
-            sse.broadcast(crate::channels::web::types::SseEvent::ExtensionStatus {
+            sse.broadcast(ironclaw_common::AppEvent::ExtensionStatus {
                 extension_name: name.to_string(),
                 status: status.to_string(),
                 message: message.map(|m| m.to_string()),
@@ -3310,7 +3310,7 @@ impl ExtensionManager {
                 }
 
                 if let Some(ref sse) = sse_manager {
-                    sse.broadcast(crate::channels::web::types::SseEvent::AuthCompleted {
+                    sse.broadcast(ironclaw_common::AppEvent::AuthCompleted {
                         extension_name: ext_name,
                         success,
                         message,