feat(epistemics): record assertions between observations and beliefs

A statement made to a mind is now three things kept apart: the observation
that it arrived, an append-only assertion (attributed actor, stance,
certainty, modality, negation, reported or quoted speech, hypothetical
status, interpretation confidence and notes), and, only for factual
assertions, one piece of evidence for a proposition belief attributed to
whoever actually stands behind it. Reported speech is attributed to the
person reported and weighed as second-hand; hypotheticals, questions and
imperatives create no belief; expressed doubt counts against; a possibly
sarcastic remark is recorded with low interpretation confidence rather
than guessed at. Interpretation is deterministic code; a model may enrich
the claim list but never sets stance or confidence.

Association no longer treats conversational turns as world events:
hypotheticals, questions, requests and consecutive turns by one speaker
cannot seed causal hypotheses.

Adds internal/epistemics, migration 0018, the assertions API, assertions
on belief pages and in belief detail, a shared end-to-end test harness
(tests/harness), end-to-end tests for eight speech patterns, and updates
the milestone test to the new provenance.

internal/cognition/handlers.go

@@ -11,6 +11,7 @@ import (
 	"github.com/andydixon/noema/internal/beliefs"
 	"github.com/andydixon/noema/internal/brain"
 	"github.com/andydixon/noema/internal/conversation"
+	"github.com/andydixon/noema/internal/epistemics"
 	"github.com/andydixon/noema/internal/graph"
 	"github.com/andydixon/noema/internal/memory"
 	"github.com/andydixon/noema/internal/predictions"
@@ -27,6 +28,7 @@ type Handlers struct {
 	Beliefs       *beliefs.Store
 	Conversations *conversation.Store
 	Graph         *graph.Store
+	Assertions    *epistemics.Store // nil disables assertion records (claims still become evidence)
 }
 
 // Register installs the handlers on a registry.