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.

migrations/0003_events.down.sql

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+DROP TABLE IF EXISTS mind_cursors;
+DROP TABLE IF EXISTS cognitive_events;

migrations/0003_events.up.sql

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+-- Immutable cognitive events and durable consumer cursors.
+
+CREATE TABLE cognitive_events (
+  id             uuid PRIMARY KEY,
+  seq            bigserial NOT NULL UNIQUE,
+  mind_id        uuid NOT NULL REFERENCES minds(id),
+  type           text NOT NULL,
+  category       text NOT NULL,
+  source         text NOT NULL,
+  occurred_at    timestamptz NOT NULL,
+  recorded_at    timestamptz NOT NULL DEFAULT now(),
+  parent_ids     uuid[] NOT NULL DEFAULT '{}',
+  causation_id   uuid,
+  correlation_id uuid,
+  salience       real NOT NULL DEFAULT 0 CHECK (salience >= 0 AND salience <= 1),
+  confidence     real NOT NULL DEFAULT 0 CHECK (confidence >= 0 AND confidence <= 1),
+  valence        real CHECK (valence IS NULL OR (valence >= -1 AND valence <= 1)),
+  arousal        real CHECK (arousal IS NULL OR (arousal >= 0 AND arousal <= 1)),
+  evidence       jsonb NOT NULL DEFAULT '[]'::jsonb,
+  summary        text NOT NULL,
+  metadata       jsonb NOT NULL DEFAULT '{}'::jsonb,
+  payload        jsonb NOT NULL DEFAULT '{}'::jsonb
+);
+CREATE INDEX cognitive_events_mind_seq_idx ON cognitive_events (mind_id, seq);
+CREATE INDEX cognitive_events_mind_type_seq_idx ON cognitive_events (mind_id, type, seq);
+CREATE INDEX cognitive_events_correlation_idx ON cognitive_events (correlation_id) WHERE correlation_id IS NOT NULL;
+CREATE INDEX cognitive_events_causation_idx ON cognitive_events (causation_id) WHERE causation_id IS NOT NULL;
+CREATE INDEX cognitive_events_parents_idx ON cognitive_events USING gin (parent_ids);
+CREATE INDEX cognitive_events_evidence_idx ON cognitive_events USING gin (evidence jsonb_path_ops);
+CREATE TRIGGER cognitive_events_append_only BEFORE UPDATE OR DELETE ON cognitive_events
+  FOR EACH ROW EXECUTE FUNCTION noema_forbid_change();
+
+-- Durable read positions for consumers (the mind runtime, consolidation, ...).
+CREATE TABLE mind_cursors (
+  mind_id    uuid NOT NULL REFERENCES minds(id),
+  consumer   text NOT NULL,
+  last_seq   bigint NOT NULL DEFAULT 0,
+  updated_at timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY (mind_id, consumer)
+);