Soften Planner change marks and move the changes chip into the margin

Washes are lighter, rounded and bled past the text, fade in when seen and
decay when they expire. Blocks inserted in front of the reader open up
instead of shoving the page down. One turn's marks fade together. MCP edits
name the caller instead of "unknown". The Planner's cursor label sits beside
its caret. The changes chip uses icons, has 24px targets, sits in the
leading margin, and its list rows are no longer tinted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>