feat: Add 'done' status with 7-day auto-close workflow

Added 'done' status to task lifecycle for verification period before completion.

**Changes:**
- Added 'done' to task_status enum (pending, open, in_progress, testing, blocked, done, completed)
- Added auto_close_at timestamp column for scheduling auto-close
- Created index for efficient cron job queries

**Workflow:**
1. Task marked 'done' → auto_close_at = NOW() + 7 days
2. Cron job runs daily to check tasks past auto_close_at
3. If no related issues → auto-close to 'completed'
4. If related issues → keep as 'done' (prevents premature closure)

**Lifecycle:**
  open → in_progress → done (7-day verification) → completed (auto-closed)

task:CF-454

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

migrations/022_add_done_status.sql

@@ -0,0 +1,42 @@
+-- Add "done" status and auto-close workflow
+--
+-- Migration: 022_add_done_status
+-- Purpose: Add "done" status for verification period before auto-closing
+-- Related: CF-454
+--
+-- Status lifecycle with done:
+--   - pending: Created, awaiting approval or dependencies
+--   - open: Approved and ready to start
+--   - in_progress: Currently being worked on
+--   - testing: Implementation complete, being tested
+--   - blocked: Stuck waiting for something
+--   - done: Implementation complete, in 7-day verification period
+--   - completed: Verified stable, auto-closed after 7 days
+--
+-- Verification period:
+--   - Tasks marked "done" wait 7 days before auto-closing to "completed"
+--   - If related issues detected during period, task stays "done" (no auto-close)
+--   - Prevents premature closure and catches regressions
+--
+-- Auto-close workflow:
+--   1. Task marked as "done" → auto_close_at = now() + 7 days
+--   2. Cron job checks daily for tasks past auto_close_at
+--   3. Semantic search detects related issues created since task marked "done"
+--   4. If no related issues → auto-close to "completed"
+--   5. If related issues found → notify, keep as "done"
+
+-- Drop existing constraint
+ALTER TABLE tasks DROP CONSTRAINT IF EXISTS tasks_status_check;
+
+-- Add new constraint with done status
+ALTER TABLE tasks ADD CONSTRAINT tasks_status_check
+    CHECK (status IN ('pending', 'open', 'in_progress', 'testing', 'blocked', 'done', 'completed'));
+
+-- Add auto_close_at column for tracking when to auto-close
+ALTER TABLE tasks ADD COLUMN IF NOT EXISTS auto_close_at TIMESTAMP WITH TIME ZONE;
+
+-- Create index for efficient cron job queries
+CREATE INDEX IF NOT EXISTS idx_tasks_auto_close ON tasks(auto_close_at) WHERE auto_close_at IS NOT NULL;
+
+-- Add comment for documentation
+COMMENT ON COLUMN tasks.auto_close_at IS 'Timestamp when task should auto-close from done to completed (7 days after marked done)';