Add round-robin posting mode

One story per interval_hours (default 4), cycling through enabled sources
in config order. Empty sources are skipped and the pointer advances to
whichever source actually posted. State persists across restarts.

Duplicate-flagged stories now get a _duplicate sentinel channel so they
stay out of the rotation pool alongside _discarded.

internal/config/config.go

@@ -44,7 +44,16 @@ type PostingConfig struct {
 	DedupCooldownHours    int `yaml:"dedup_cooldown_hours"`
 	// DailyCapTotal is the hard global cap on posts across ALL channels in a
 	// rolling 24-hour window. 0 disables the cap.
-	DailyCapTotal int `yaml:"daily_cap_total"`
+	DailyCapTotal int              `yaml:"daily_cap_total"`
+	RoundRobin    RoundRobinConfig `yaml:"round_robin"`
+}
+
+// RoundRobinConfig switches Pete from immediate-on-classify posting to a
+// paced rotation: one story per IntervalHours, picking the next source in
+// config order that has a postable story (skip-and-advance, newest-first).
+type RoundRobinConfig struct {
+	Enabled       bool `yaml:"enabled"`
+	IntervalHours int  `yaml:"interval_hours"`
 }
 
 type StorageConfig struct {
@@ -160,6 +169,9 @@ func (c *Config) applyDefaults() {
 	if c.Posting.DedupCooldownHours == 0 {
 		c.Posting.DedupCooldownHours = 48
 	}
+	if c.Posting.RoundRobin.IntervalHours == 0 {
+		c.Posting.RoundRobin.IntervalHours = 4
+	}
 	if c.Storage.RecentWindowHours == 0 {
 		c.Storage.RecentWindowHours = 24
 	}