fix(canvas): cap CONTEXT.md description textarea height

[adboio]

Problem

When creating a channel's CONTEXT.md with an agent, the description textarea in the "Create your context.md" dialog grows without limit as you type. Because quill's Textarea auto-grows to fit its content (field-sizing: content) and has no max-height, a long enough description expands the modal past the viewport and pushes the footer's Create button off-screen — leaving ⌘/Ctrl+Enter as the only way to submit (which most users won't discover).

Why: reported by a user hitting this in the Channels (project-bluebird) flow — the submit button becomes unclickable for longer descriptions.

Changes

• Cap the description Textarea at 40vh with overflow-y-auto so it scrolls internally instead of expanding the dialog. The field is shared by both the describe-mode dialog and the create-mode step-two dialog, so both are fixed.

How did you test this?

• Manual reasoning only — I don't have the app running in this environment. The change is a className on a component that already forwards className to the underlying quill primitive; no behavior beyond height/overflow changes.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[trunk-io]

✨ Submitted to Merge by @adboio. It will be added to the merge queue once all branch protection rules pass, there are no merge conflicts with the target branch, and impacted targets for the current PR head commit have been uploaded. See more details here.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit d1721a7.}

[github-actions]

Trivial CSS fix adding a max-height cap and scroll to a textarea. No logic, no data, no risk.