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.

web/templates/pages/personality.html

@@ -4,7 +4,7 @@
   <div><h1>{{$m.DisplayName}} <span class="muted small">personality</span></h1><p class="muted">Version {{$m.PersonalityVersion}}{{if .Data.Template}} · based on template <strong>{{.Data.Template.Name}}</strong>{{end}}. Every save creates a new version; nothing is overwritten.</p></div>
   <div class="row"><a class="btn small" href="/minds/{{$m.Slug}}/personality.json">Export JSON</a></div>
 </div>
-<nav class="tabs" aria-label="Mind sections"><a href="/minds/{{$m.Slug}}">Overview</a><a href="/minds/{{$m.Slug}}/personality" aria-current="page">Personality</a></nav>
+{{template "mind_tabs" (dict "M" $m "Active" "personality")}}
 {{$admin := .Principal.User.Role.AtLeast "administrator"}}
 <section class="grid cols-3-1">
   <div>