docs(planning): add spec and plan for planning merge-all feature

Covers subtask visibility fix, aggregated diff viewer, and single
Merge-all action with VS-Code-assisted conflict resolution.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/settings.local.json

@@ -6,7 +6,10 @@
       "mcp__plugin_context-mode_context-mode__batch_execute",
       "mcp__plugin_context-mode_context-mode__execute",
       "mcp__plugin_context7_context7__query-docs",
-      "mcp__plugin_context-mode_context-mode__search"
+      "mcp__plugin_context-mode_context-mode__search",
+      "Bash(git fetch *)",
+      "PowerShell(cmdkey *)",
+      "mcp__plugin_context7_context7__resolve-library-id"
     ]
   }
 }