refactor: extract AppEvent to crates/ironclaw_common (#1615)

* 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]>

* refactor: add AppEvent::event_type() helper, deduplicate match blocks

Address Gemini review: extract the variant→string match into a single
method on AppEvent, replacing the duplicated 22-arm matches in sse.rs
and types.rs.

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

* refactor: rename leftover sse vars/tests to match AppEvent rename

Address Copilot review: rename sse_event vars to app_event in
orchestrator/api.rs and ws.rs, rename test functions from
test_ws_server_from_sse_* to test_ws_server_from_app_event_*, and
update stale SSE comments.

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

* refactor: add Deserialize to AppEvent, round-trip test, fix stale comments

Address zmanian review:
- Add Deserialize derive to AppEvent so downstream consumers can
  deserialize incoming events
- Add event_type_matches_serde_type_field test that round-trips every
  variant through serde and asserts event_type() matches the serialized
  "type" field — catches drift between serde renames and the manual match
- Add round_trip_deserialize test for basic Serialize/Deserialize parity
- Update remaining "SSE" references in comments across server.rs,
  manager.rs, ws_gateway_integration.rs, and worker/job.rs

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

---------

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

Cargo.lock

@@ -3428,6 +3428,7 @@ dependencies = [
  "hyper-util",
  "iana-time-zone",
  "insta",
+ "ironclaw_common",
  "ironclaw_safety",
  "json5",
  "libsql",
@@ -3485,6 +3486,14 @@ dependencies = [
  "zip",
 ]
 
+[[package]]
+name = "ironclaw_common"
+version = "0.1.0"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "ironclaw_safety"
 version = "0.1.0"