fix: Fix context window size and redesign usage indicator

[charlesvien]

Problem

Closes #1323

Context window size was underreported by the SDK (returning 200K for 1M-context models), causing inaccurate usage percentages and premature compaction warnings.

Changes

1. Use gateway-reported context window as floor, only adopt SDK value when larger
2. Persist lastContextWindowSize across prompt() calls so it survives turn boundaries
3. Replace text-only percentage indicator with circular progress ring in session footer
4. Emit live usage_update during streaming (on assistant messages, not just on result)
5. Show context percentage in compact boundary markers
6. Remove old getDefaultContextWindow hardcoded lookup in favor of gateway model data

How did you test this?

Manually

[charlesvien]

• fix: Fix context window size and redesign usage indicator #1328 👈 (View in Graphite)
• feat: Upgrade Claude adapter to ACP SDK v0.22.2 #1330
• main

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[charlesvien]

Merge activity

• Mar 25, 5:48 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 25, 5:49 PM UTC: Graphite rebased this pull request as part of a merge.
• Mar 25, 5:56 PM UTC: @charlesvien merged this pull request with Graphite.