[bugfix][#966] Compact Stop Button Width in Plan Webview

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[bugfix][#966] Compact Stop Button Width in Plan Webview

Summary

Fixes the visually over-wide terminal Stop button in the Plan webview by removing unintended inherited equal-width flex behavior.

Changes

CSS Fix (vscode/webview/plan/styles.css)

• Removed global equal-width flex rules from .widget-button
• Added scoped rule .widget-buttons .widget-button for action-row equal-width sizing
• Added intrinsic width rules (flex: 0 0 auto; min-width: auto) to .terminal-stop-button

Documentation Updates

• vscode/webview/plan/styles.md: Documented that .widget-button provides shared visual style only; equal-width behavior applies only in .widget-buttons context; .terminal-stop-button uses intrinsic width
• vscode/test/playwright/test-session-append.md: Documented the hard assertion for stop-width regression check

Test Update

• vscode/test/playwright/test-session-append.js: Added hard regression assertion comparing terminal stop button width vs action-row button width; throws error if stop button is not narrower

Verification

• Build passes (make vscode-plugin succeeds)
• Terminal Stop button is now content-sized (not stretched)
• Action-row buttons in .widget-buttons remain equal-width
• Playwright test includes deterministic regression check for stop-width behavior

closes #966

Issue 966 resolved