feat(KB-034): add task archive/unarchive functionality

- Add 'archived' column to task store with archiveTask and unarchiveTask methods
- Add CLI commands: kb task archive <id> and kb task unarchive <id>
- Add pi extension tools for archive and unarchive operations
- Add dashboard API endpoints POST /api/tasks/:id/archive and /unarchive
- Add Archived column to board UI with archive/unarchive buttons
- Prevent drag-drop into archived column, add visual distinction
- Include duplicateTask from concurrent branch in merge resolution

packages/cli/src/__tests__/extension.test.ts

@@ -84,6 +84,8 @@ describe("kb pi extension", () => {
         "kb_task_import_github",
         "kb_task_import_github_issue",
         "kb_task_browse_github_issues",
+        "kb_task_archive",
+        "kb_task_unarchive",
       ];
 
       for (const name of expected) {

packages/cli/src/bin.ts

@@ -39,7 +39,7 @@ if (isBunBinary) {
 
 // Dynamic imports so the pi-coding-agent config module sees PI_PACKAGE_DIR
 const { runDashboard } = await import("./commands/dashboard.js");
-const { runTaskCreate, runTaskList, runTaskMove, runTaskMerge, runTaskUpdate, runTaskLog, runTaskShow, runTaskAttach, runTaskPause, runTaskUnpause, runTaskImportFromGitHub, runTaskDuplicate } = await import("./commands/task.js");
+const { runTaskCreate, runTaskList, runTaskMove, runTaskMerge, runTaskUpdate, runTaskLog, runTaskShow, runTaskAttach, runTaskPause, runTaskUnpause, runTaskImportFromGitHub, runTaskDuplicate, runTaskArchive, runTaskUnarchive } = await import("./commands/task.js");
 
 const HELP = `
 kb — AI-orchestrated task board
@@ -56,6 +56,8 @@ Usage:
   kb task log <id> <message>          Add a log entry
   kb task merge <id>                  Merge an in-review task and close it
   kb task duplicate <id>              Duplicate a task (creates copy in triage)
+  kb task archive <id>                Archive a done task
+  kb task unarchive <id>              Unarchive an archived task
   kb task attach <id> <file>          Attach a file to a task
   kb task pause <id>                  Pause a task (stops all automation)
   kb task unpause <id>                Unpause a task (resumes automation)
@@ -72,7 +74,7 @@ Options:
   --interactive, -i          Interactive mode for issue selection
   --help, -h                 Show this help
 
-Columns: triage, todo, in-progress, in-review, done
+Columns: triage, todo, in-progress, in-review, done, archived
 Supported file types: png, jpg, gif, webp, txt, log, json, yaml, yml, toml, csv, xml
 
 The AI engine uses pi (github.com/badlogic/pi-mono) for agent sessions.
@@ -174,6 +176,18 @@ async function main() {
             await runTaskDuplicate(id);
             break;
           }
+          case "archive": {
+            const id = args[2];
+            if (!id) { console.error("Usage: kb task archive <id>"); process.exit(1); }
+            await runTaskArchive(id);
+            break;
+          }
+          case "unarchive": {
+            const id = args[2];
+            if (!id) { console.error("Usage: kb task unarchive <id>"); process.exit(1); }
+            await runTaskUnarchive(id);
+            break;
+          }
           case "attach": {
             const id = args[2], file = args[3];
             if (!id || !file) {

packages/cli/src/commands/task.ts

@@ -296,6 +296,24 @@ export async function runTaskDuplicate(id: string) {
   console.log();
 }
 
+export async function runTaskArchive(id: string) {
+  const store = await getStore();
+  const task = await store.archiveTask(id);
+
+  console.log();
+  console.log(`  ✓ Archived ${task.id} → ${COLUMN_LABELS[task.column]}`);
+  console.log();
+}
+
+export async function runTaskUnarchive(id: string) {
+  const store = await getStore();
+  const task = await store.unarchiveTask(id);
+
+  console.log();
+  console.log(`  ✓ Unarchived ${task.id} → ${COLUMN_LABELS[task.column]}`);
+  console.log();
+}
+
 export async function runTaskImportGitHubInteractive(
   ownerRepo: string,
   options: TaskImportOptions = {}

packages/cli/src/extension.ts

@@ -371,6 +371,63 @@ export default function kbExtension(pi: ExtensionAPI) {
     },
   });
 
+  // ── kb_task_archive ───────────────────────────────────────────────
+
+  pi.registerTool({
+    name: "kb_task_archive",
+    label: "KB: Archive Task",
+    description:
+      "Archive a done task (move from done → archived). " +
+      "Archived tasks are preserved for historical reference but moved out of the main board view.",
+    promptSnippet: "Archive a done kb task (moves to archived column)",
+    promptGuidelines: [
+      "Use to clean up old completed tasks from the done column",
+      "Only tasks in the 'done' column can be archived",
+      "Archived tasks can be unarchived later if needed",
+    ],
+    parameters: Type.Object({
+      id: Type.String({ description: "Task ID to archive (e.g. KB-001). Must be in 'done' column." }),
+    }),
+
+    async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+      const store = await getStore(ctx.cwd);
+      const task = await store.archiveTask(params.id);
+
+      return {
+        content: [{ type: "text", text: `Archived ${task.id} → ${COLUMN_LABELS[task.column]}` }],
+        details: { taskId: task.id, column: task.column },
+      };
+    },
+  });
+
+  // ── kb_task_unarchive ─────────────────────────────────────────────
+
+  pi.registerTool({
+    name: "kb_task_unarchive",
+    label: "KB: Unarchive Task",
+    description:
+      "Unarchive an archived task (move from archived → done). " +
+      "Restores the task to the done column.",
+    promptSnippet: "Unarchive a kb task (restores to done column)",
+    promptGuidelines: [
+      "Use to restore an archived task back to the done column",
+      "Only tasks in the 'archived' column can be unarchived",
+    ],
+    parameters: Type.Object({
+      id: Type.String({ description: "Task ID to unarchive (e.g. KB-001). Must be in 'archived' column." }),
+    }),
+
+    async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+      const store = await getStore(ctx.cwd);
+      const task = await store.unarchiveTask(params.id);
+
+      return {
+        content: [{ type: "text", text: `Unarchived ${task.id} → ${COLUMN_LABELS[task.column]}` }],
+        details: { taskId: task.id, column: task.column },
+      };
+    },
+  });
+
   // ── kb_task_import_github ─────────────────────────────────────────
 
   pi.registerTool({