feat(pi): add configuration options for todo widget

[autt-carrum]

Configurable Pi progress widget

The Pi extension's plannotator-progress widget renders one line per checklist item during plan execution. Long plans crowd the terminal, and there's no way to shrink it today. This PR adds two config-only keys in ~/.plannotator/config.json that reshape the widget without touching the underlying checklist.

New settings

• widgetStyle — "default" (unchanged behavior) · "compact" (single 📋 done/total · next: <step> line, or · all done) · positive integer N (cap the widget at N item lines, preferring remaining steps and backfilling with the most recent completed ones, with a muted overflow line summarizing what was dropped: … +K more todo, … +K done, or … +K done, +M todo).
• widgetMoveCompletedToEnd — boolean, default false. When true, the widget renders completed items after remaining ones.

Both keys are config-file only — no PLANNOTATOR_WIDGET_* env override. Invalid widgetStyle values (zero, negatives, non-integer numbers, unknown strings) silently fall back to "default" so a hand-edited config never blanks the widget.

Widget-only

The status label, the phase-entry ${todoList} template, the injected per-turn todo-status message, and the pi-todos mirror all keep reading the raw checklist in its original order. widgetMoveCompletedToEnd has no visible effect under widgetStyle: "compact" (nothing to reorder).

Changes

File	Change
packages/shared/config.ts	Add widgetStyle / widgetMoveCompletedToEnd to PlannotatorConfig; add resolveWidgetStyle() returning a WidgetStyle tagged union (default / compact / limit) and resolveWidgetMoveCompletedToEnd()
apps/pi-extension/index.ts	Rewrite updateWidget to dispatch on style, partition and slice items for the limit path, emit the overflow line
packages/shared/config.test.ts	Resolver tests: defaults, valid values, invalid fallbacks for both keys
apps/pi-extension/README.md	New "Configuring the progress widget" section with JSON examples per mode
AGENTS.md	Documented under Config-only settings

Test plan

• bun test packages/shared/config.test.ts — 86 pass, 0 fail.
• Manual: verified the compact single-line render (📋 20/20 · all done at completion) with a 20-step smoke plan.
• Manual: verified the limit path (widgetStyle: 4, widgetMoveCompletedToEnd: true) with a 12-step plan — widget capped at 4 lines with backfilled completed items rendered below remaining ones and … +K done, +M more todo overflow.
• Regression: with both keys omitted, widget renders exactly as today.

Backwards compatibility

Fully backwards compatible. Both keys are optional; unset config behaves identically to today.