Add entity-aware fact resolution against Nexus (Vikunja #279)

facts gain a Subject/EntityID/ResolutionState triple and an async
enrichment worker that resolves free-text subjects to canonical Nexus
entity_ids, mirroring Praxis's enrichment-worker pattern. Ambiguous or
unreachable Nexus never guesses an entity_id — the fact stays pending
or terminal-ambiguous instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ghELqYhZNLub2TXGMazqA

internal/store/store.go

@@ -40,8 +40,32 @@ type Fact struct {
 	Source     string // tap:* | infer:* | poll:* | ambient | promote | feedback
 	Confidence float64
 	VoidsID    sql.NullInt64
+
+	// Subject, EntityID, ResolutionState — entity-aware memory (Vikunja #279).
+	// Subject is the free-text "who/what this fact is about" supplied at write
+	// time by WriteFactAboutSubject; empty means the fact isn't about a
+	// resolvable entity (ResolutionState stays "none"). EntityID is the
+	// canonical Nexus entity_id once the enrichment worker resolves Subject.
+	Subject         string
+	EntityID        sql.NullString
+	ResolutionState FactResolutionState
 }
 
+// FactResolutionState — where a fact's Subject stands in Nexus entity
+// resolution. "none" = no subject given (most facts). "pending" = subject
+// given, not yet resolved. Terminal states: "resolved", "ambiguous" (Nexus
+// returned candidates, not stored — matches the ecosystem's ambiguity-blocks
+// invariant), "not_found".
+type FactResolutionState string
+
+const (
+	ResolutionNone      FactResolutionState = "none"
+	ResolutionPending   FactResolutionState = "pending"
+	ResolutionResolved  FactResolutionState = "resolved"
+	ResolutionAmbiguous FactResolutionState = "ambiguous"
+	ResolutionNotFound  FactResolutionState = "not_found"
+)
+
 // Bucket — presence hysteresis state.
 type Bucket string