loop: add rule trace/explanation engine

ExplainTick and ExplainGate produce a full TickTrace for every tick,
recording per-rule: predicate result, gate result, first gate blocker,
gate detail (snooze/cooldown/presence etc.), and win/loss info.

IPC layer: new MethodTickTrace, DTOs (TickTrace, RuleTrace, GateDetail),
dispatch, client proxy, and CoreAPI interface method.

Daemon: tickLoop caches the latest trace; daemonAPI wraps storeAPI
with a TickTrace override that returns the cached trace.

Tests: 15 new tests for ExplainGate (all blockers, bypass conditions,
ordering) and ExplainTick (nothing fires, one fires, tiebreak,
winner/lost_to recording, gate-blocked recording).

internal/ipc/api.go

@@ -240,6 +240,42 @@ type CoreAPI interface {
 	LookupTool(ctx context.Context, name string) (Tool, error)
 	ListTools(ctx context.Context, status string) ([]Tool, error)
 	RevertFact(ctx context.Context, key string) (int64, error)
+
+	// TickTrace returns the most recent tick's rule trace. The daemon caches
+	// this after every tick; the store adapter returns an error (trace is not
+	// persisted — it's a daemon-level cache).
+	TickTrace(ctx context.Context) (TickTrace, error)
+}
+
+// --- Rule trace / explanation DTOs ---
+
+// RuleTrace — per-rule evaluation result for one tick.
+type RuleTrace struct {
+	RuleName        string     `json:"rule_name"`
+	Severity        int        `json:"severity"`
+	PredicateResult bool       `json:"predicate_result"`
+	GateResult      bool       `json:"gate_result"`
+	GateBlockedBy   string     `json:"gate_blocked_by,omitempty"`
+	GateDetail      GateDetail `json:"gate_detail,omitempty"`
+	WasSelected     bool       `json:"was_selected"`
+	LostTo          string     `json:"lost_to,omitempty"`
+}
+
+// GateDetail — snapshot of the values the gate checked.
+type GateDetail struct {
+	SnoozeUntil      *time.Time `json:"snooze_until,omitempty"`
+	CooldownUntil    *time.Time `json:"cooldown_until,omitempty"`
+	QuietHours       bool       `json:"quiet_hours"`
+	CalendarBusy     bool       `json:"calendar_busy"`
+	Presence         string     `json:"presence"`
+	InertKeysMissing []string   `json:"inert_keys_missing,omitempty"`
+}
+
+// TickTrace — snapshot of one tick's rule evaluations.
+type TickTrace struct {
+	Now    time.Time   `json:"now"`
+	Winner string      `json:"winner"`
+	Rules  []RuleTrace `json:"rules"`
 }
 
 // ErrToolNotFound — no tool row with this name (re-exported store sentinel for