docs(sessions): awwwards-grade experience design for chat sessions

Companion to levelcode-chat-sessions-design.md (the persistence spine). Where
the spine keeps the UI 'classic, minimal', this specs the experience layer:
the Sessions panel + rich session cards (activity sparkline, files-touched
chips, state pills), a keyboard-first fuzzy switcher, search over what a
session DID (files/commands, not just titles), the resume shared-element
morph, the honest summarize-to-fit sheet, fork-a-session, and the motion/
a11y/theming system — grounded in the shipped 'classic' (--cc-*) tokens and
tested via the same static-invariant approach as the rest of the webview.

Competitive target: beat Copilot's dropdown and Cursor's title-only list.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ndemianc