add pi task extension

pi/extensions/pi-task/README.md

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+# pi-task
+
+A small, session-scoped task list for Pi. It adds one agent tool, `task`, a
+compact widget above the editor, and a progress indicator in the footer.
+
+The tool accepts the complete ordered task list, like Claude's todo tool or
+Codex's plan tool. The agent adds, updates, removes, or reorders tasks by
+submitting a new list. Omitting the list reads the current state; submitting an
+empty list clears it.
+
+Statuses are `pending`, `in_progress`, and `completed`. At most one task can be
+in progress at a time.
+
+State is stored in Pi tool-result details. It survives reloads and compaction,
+follows session branches correctly, and does not create project files. A new Pi
+session starts with an empty list.
+
+While work is active, the widget uses themed symbols and colors to distinguish
+the current task (`●`) from pending tasks (`○`). The footer shows compact
+overall progress. When every task is complete, Pi displays a success
+notification and removes both the widget and footer indicator to reclaim space.
+The completed list remains available in session history.
+
+Run `/tasks` to refresh the task UI manually.