extensions: unify todo directory resolution across idle-compact and todos

[benvinegar]

Problem

idle-compact.ts and todos.ts resolve todo directories differently:

• idle-compact.ts derives path from PI_TODO_PATH or join('.pi','todos'), then applies process.cwd().
• todos.ts has its own getTodosDir()/label logic and default semantics.

This can cause idle compaction to think there are no active todos when the todo tool is actually using a different directory.

Proposed solution

• Extract shared todo-dir resolution utility used by both extensions.
• Define one canonical resolution order:
  1. PI_TODO_PATH absolute/relative
  2. default .pi/todos relative to canonical runtime cwd (or explicit home-based path)
• Add tests proving both extensions see the same effective todo dir under different cwd/env combinations.

Helpful context

• pi/extensions/idle-compact.ts: TODO_DIR, hasInProgressTodos() path resolution via process.cwd().
• pi/extensions/todos.ts: TODO_PATH_ENV, TODO_DIR_NAME, getTodosDir(process.cwd()).