[Feature] Redesign session right-panel Context tab against DESIGN.md

[Astro-Han]

Goal

Redesign the right-panel Context / 上下文 tab against DESIGN.md, separately from the slice 11b session-view rewrite. Captures the deferred scope so it does not get lost.

Background

#440 slice 11b rewrites the right-panel chrome (tab bar, overview, status, connection health, review filter, terminal sub-tabs). #154 explicitly excludes Context redesign from that scope:

Do not redesign Context / 上下文, Terminal / 终端, Review / 审查, or diagnostic surfaces in this issue.

Slice 11b.2 will align the Context tab strip with the new chrome tokens (spacing, font, border) but leave the internal information architecture untouched. This issue tracks the remaining redesign.

In-scope files

• packages/app/src/components/session/session-context-tab.tsx (~347 lines) — UI panel
• packages/app/src/components/session/session-context-breakdown.ts — token estimation logic
• packages/app/src/components/session/session-context-format.ts — formatting helpers
• packages/app/src/components/session/session-context-metrics.ts — provider model metadata

What a good result looks like

• Information architecture re-evaluated against DESIGN.md: is the stacked breakdown bar the right primary visual, or a denser table / inline metric strip?
• Tokens-by-message list aligned with Cards or chat-row patterns from DESIGN.md.
• Color usage within the Restrained palette; no per-segment brand color signals.
• Formatting helpers reviewed: still need locale-aware percent / time formatting?

Out of scope

• Anything outside the four files above
• Backend token-accounting changes
• Provider / model picker behavior

Dependencies

• Wait for slice 11b.2 to land in [Feature] Rewrite packages/ui + packages/app visual layer against PawWork design system (incremental PR series) #440 so the right-panel chrome is stable.