feat: add immutable cognitive event store, live streams and event explorer

Phase 3. Events are the append-only record of everything a mind does
(ADR-0002). pkg/events defines the closed type vocabulary, categories,
evidence references and the observation payload with explicit trust
levels. The store validates and appends inside the caller's transaction,
publishes to a bounded fan-out bus only after commit, queries by seq
cursor with type/category/source/salience/confidence/time/search
filters, and walks provenance: cause, parents, children, cited evidence,
derived events and correlation groups. Durable per-consumer cursors never
move backwards.

Server-Sent Events streams (per mind and global) subscribe before
replaying history after Last-Event-ID so nothing is missed, apply the
same filter vocabulary live, and heartbeat. Operators can inject
observations tagged with operator trust for demos and tests.

UI: live thought stream with filters and paging, forensic event page
with causal chain and evidence navigation, dashboard important events.
Personality and status changes now leave operator_action events in the
mind's own history. CLI: event inspect/list/tail, observe.

internal/app/app.go

@@ -13,6 +13,7 @@ import (
 	"github.com/andydixon/noema/internal/audit"
 	"github.com/andydixon/noema/internal/auth"
 	"github.com/andydixon/noema/internal/config"
+	"github.com/andydixon/noema/internal/events"
 	"github.com/andydixon/noema/internal/minds"
 	"github.com/andydixon/noema/internal/observability"
 )
@@ -26,6 +27,7 @@ type Deps struct {
 	Audit      *audit.Logger
 	Metrics    *observability.Metrics
 	Minds      *minds.Store
+	Events     *events.Store
 	Migrations fs.FS
 	Started    time.Time
 }