nudge: add digest/batching mode for care nudges

When enabled, eligible nudges (sev ≤ ceiling) are queued in memory
instead of sent immediately. Every 'window' duration (or when
'max_items' reached), the queue is flushed as a single digest
notification with concatenated bodies.

Config:
  digest:
    enabled: true           # default false
    window: 30m             # flush window (default 30m)
    max_items: 5            # flush at this count (default 5)
    severity_ceiling: 2     # max sev batched (default 2; sev3+ bypass)

Changes:
- config.go: add DigestConfig struct with defaults
- tick.go: QueuedNudge type, digest queue/flush in TickLoop,
  shouldQueue/maybeFlush/flushDigest helpers
- main.go: pass cfg.Digest to newTickLoop
- tick_test.go: 6 new tests covering queue, flush, bypass, dedup

internal/config/config.go

@@ -115,6 +115,10 @@ type Config struct {
 	// fact independently; both the schedule AND the toggle activate quiet.
 	// nil ⇒ quiet hours only activate via the voice toggle.
 	QuietHours *QuietHoursConfig `json:"quiet_hours,omitempty"`
+
+	// Digest — notification batching / digest mode. nil ⇒ digest disabled
+	// (every nudge is sent as it fires — legacy behaviour).
+	Digest *DigestConfig `json:"digest,omitempty"`
 }
 
 // QuietHoursConfig — a recurring daily quiet-window. Times are local to the
@@ -185,6 +189,18 @@ type ToolConfig struct {
 	Destructive bool     `json:"destructive,omitempty"`
 }
 
+// DigestConfig — notification batching / digest mode. When enabled, eligible
+// nudges (severity ≤ SeverityCeiling) are queued in memory instead of sent
+// immediately. Every Window duration (or when MaxItems reached), the queue is
+// flushed as a single digest notification. nil ⇒ digest disabled (legacy
+// behaviour — every nudge is sent as it fires).
+type DigestConfig struct {
+	Enabled         bool     `json:"enabled,omitempty"`
+	Window          Duration `json:"window,omitempty"`           // e.g. "30m"
+	MaxItems        int      `json:"max_items,omitempty"`        // flush at this count
+	SeverityCeiling int      `json:"severity_ceiling,omitempty"` // max sev batched
+}
+
 // PhraserConfig — the LLM-backed phraser seam. The daemon spawns llama-server
 // as a managed subprocess and sends chat-completion requests to phrase nudge
 // and reminder messages. nil ⇒ the template-based Stub is used instead.
@@ -315,6 +331,19 @@ func (c *Config) applyDefaults() {
 		}
 	}
 
+	if c.Digest == nil {
+		c.Digest = &DigestConfig{Enabled: false}
+	}
+	if c.Digest.Window == 0 {
+		c.Digest.Window = Duration(30 * time.Minute)
+	}
+	if c.Digest.MaxItems == 0 {
+		c.Digest.MaxItems = 5
+	}
+	if c.Digest.SeverityCeiling == 0 {
+		c.Digest.SeverityCeiling = 2
+	}
+
 	if c.Voice != nil {
 		if c.Voice.RouterThreshold <= 0 {
 			c.Voice.RouterThreshold = DefaultRouterThreshold