feat: add annotations, undo system, and schema updates

Add annotations as JSON column on tasks table with Annotate/Denotate
methods and CLI commands. Add undo system backed by change_log with
lightweight undo_stack table (capped at 10 entries). All mutating CLI
commands (add, done, delete, modify, start, stop) now record undo
entries. Undo restores prior task state from change_log data.

Schema changes (in v1 migration):
- annotations TEXT column on tasks
- undo_stack table
- annotations field in change_log triggers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/database.go

@@ -112,7 +112,8 @@ func runMigrations() error {
 					
 					recurrence_duration INTEGER,
 					parent_uuid TEXT,
-					
+					annotations TEXT DEFAULT NULL,
+
 					FOREIGN KEY (parent_uuid) REFERENCES tasks(uuid) ON DELETE CASCADE
 				);
 
@@ -209,6 +210,15 @@ func runMigrations() error {
 				CREATE INDEX idx_refresh_tokens_hash ON refresh_tokens(token_hash);
 				CREATE INDEX idx_refresh_tokens_user ON refresh_tokens(user_id);
 
+				-- Undo stack (local-only, references change_log entries)
+				CREATE TABLE undo_stack (
+					id              INTEGER PRIMARY KEY AUTOINCREMENT,
+					created_at      INTEGER NOT NULL,
+					op_type         TEXT NOT NULL,
+					task_uuid       TEXT NOT NULL,
+					change_log_id   INTEGER NOT NULL
+				);
+
 				-- Triggers to populate change_log
 				CREATE TRIGGER track_task_create AFTER INSERT ON tasks
 				BEGIN
@@ -241,10 +251,11 @@ func runMigrations() error {
 						CASE WHEN NEW.until_date IS NOT NULL THEN 'until: ' || NEW.until_date || CHAR(10) ELSE '' END ||
 						CASE WHEN NEW.recurrence_duration IS NOT NULL THEN 'recurrence: ' || NEW.recurrence_duration || CHAR(10) ELSE '' END ||
 						CASE WHEN NEW.parent_uuid IS NOT NULL THEN 'parent_uuid: ' || NEW.parent_uuid || CHAR(10) ELSE '' END ||
-						(SELECT CASE WHEN COUNT(*) > 0 
+						CASE WHEN NEW.annotations IS NOT NULL THEN 'annotations: ' || NEW.annotations || CHAR(10) ELSE '' END ||
+						(SELECT CASE WHEN COUNT(*) > 0
 							THEN 'tags: ' || GROUP_CONCAT(tag, ',') || CHAR(10)
-							ELSE '' 
-						END 
+							ELSE ''
+						END
 						FROM (SELECT tag FROM tags WHERE task_id = NEW.id ORDER BY tag))
 					);
 				END;
@@ -280,10 +291,11 @@ func runMigrations() error {
 						CASE WHEN NEW.until_date IS NOT NULL THEN 'until: ' || NEW.until_date || CHAR(10) ELSE '' END ||
 						CASE WHEN NEW.recurrence_duration IS NOT NULL THEN 'recurrence: ' || NEW.recurrence_duration || CHAR(10) ELSE '' END ||
 						CASE WHEN NEW.parent_uuid IS NOT NULL THEN 'parent_uuid: ' || NEW.parent_uuid || CHAR(10) ELSE '' END ||
-						(SELECT CASE WHEN COUNT(*) > 0 
+						CASE WHEN NEW.annotations IS NOT NULL THEN 'annotations: ' || NEW.annotations || CHAR(10) ELSE '' END ||
+						(SELECT CASE WHEN COUNT(*) > 0
 							THEN 'tags: ' || GROUP_CONCAT(tag, ',') || CHAR(10)
-							ELSE '' 
-						END 
+							ELSE ''
+						END
 						FROM (SELECT tag FROM tags WHERE task_id = NEW.id ORDER BY tag))
 					);
 				END;