Clear the 18 known contrast entries added by the measured axe check #138

#120 taught the Storybook axe sweep to measure text contrast itself where axe can't, for example on translucent or stacked backgrounds. The new check found problems in stories that already existed. They were added to e2e/stories-a11y.known.json so #120 could land, and this issue tracks clearing them.

There are 18 entries: 9 stories, each in light and dark.

Measured below 4.5:1 (a real failure)

• primitives-sidebar-panel--channels-panel: the sidebar badge measures 4.38:1 on the stacked washes.

Unmeasured: the check can't find the colour under the text

• chat-prompt-input--command-suggestions
• chat-prompt-input--disabled
• chat-prompt-input--long-draft
• chat-prompt-input--replying
• chat-prompt-input--suggestions-open
• chat-prompt-input--with-tokens
• primitives-avatar--orbit
• primitives-avatar--stack-with-status

Done when

• The badge reaches 4.5:1 in both themes.
• Each unmeasured story either measures and passes, or has a short note in the known file saying why it can't be measured.
• The 18 entries are gone from e2e/stories-a11y.known.json.