feat(phase978): guild mesh track regression chain journey stub

Add runGuildMeshTrackRegressionChainJourney for phases 956–977 chain
verification. Non-breaking agentctl harness only.

Co-authored-by: Cursor <[email protected]>

headless.js

@@ -1817,6 +1817,27 @@ class HeadlessSession {
     return { multi, federation }
   }
 
+  /** Journey stub: guild mesh production track regression chain 956–977 (Phase 978). */
+  async runGuildMeshTrackRegressionChainJourney (guildId) {
+    if (!this.platform) throw new Error('headless not started')
+    const gid = guildId || this.platform.guild?.guild?.id
+    if (!gid) throw new Error('no guild')
+    const {
+      listGuildMeshTrackRegressionPhases,
+      buildGuildMeshTrackRegressionChainSnapshot,
+      GUILD_MESH_TRACK_REGRESSION_CHAIN_VERSION
+    } = require('pearcord-platform/guild-mesh-production-track-regression-chain')
+    const phases = listGuildMeshTrackRegressionPhases()
+    const snapshot = buildGuildMeshTrackRegressionChainSnapshot()
+    await this.refreshView()
+    return {
+      guildId: gid,
+      phases,
+      snapshot,
+      chainVersion: GUILD_MESH_TRACK_REGRESSION_CHAIN_VERSION
+    }
+  }
+
   /** Journey stub: guild mesh stability stats facade + mixin import chain (Phase 977). */
   async runGuildMeshStabilityStatsFacadeJourney (guildId) {
     if (!this.platform) throw new Error('headless not started')