refactor(cli): redesign skip grouping in console output

[sjawhar]

Summary

Replaces the threshold-based skip collapsing (>20 consecutive → collapse) with consistent category-aware grouping:

• Always collapse 2+ consecutive same-category skips — removed the 20-stage threshold that caused inconsistent display
• Split groups by category — cached, blocked, and cancelled stages get separate collapsed lines instead of a generic group
• Category-specific labels — groups show "3 cached" or "2 blocked" instead of "3 stages not run"
• Dim single skip lines — individual cached/blocked/cancelled stages have consistent visual weight with collapsed groups

Before

[  1– 63/175] ○ 63 stages not run          ← collapsed, generic label
[ 66/175]     ○ compute_task_weights cached ← bright, individual
[ 67/175]     ○ wrangle_bootstrap    cached ← bright, individual
[ 68/175]     ○ wrangle_logistic     cached ← bright, individual

After

[  1– 63/175] ○ 63 cached                  ← collapsed, specific label
[ 66– 68/175] ○ 3 cached                   ← collapsed (was 3 individuals)

Design doc: docs/plans/2026-02-13-skip-grouping-redesign.md

[Copilot]

Pull request overview

This PR refactors Pivot’s console output for skipped stages by replacing the old threshold-based collapsing with consistent, category-aware grouping, improving readability and consistency in both static and live console sinks.

Changes:

• Collapses 2+ consecutive skipped stages only when they share the same category (cached/blocked/cancelled), removing the previous 20-stage threshold.
• Updates skip-group formatting to include category-specific labels (e.g., “3 cached”) and dims individual skip lines for consistent visual weight.
• Refreshes and expands sink tests to cover single-skip behavior, same-category collapsing, and category boundary cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/pivot/src/pivot/engine/sinks.py	Implements same-category skip grouping (2+ collapses), category-labeled group lines, and dimmed single-skip output.
packages/pivot/tests/engine/test_sinks.py	Updates existing assertions and adds/adjusts tests to match the new grouping + labeling behavior.
docs/plans/2026-02-13-skip-grouping-redesign.md	Adds an implementation plan documenting the intended behavior and test changes.