fix(agent-builder): don't auto-send prompt when opening the dock

[dmarticus]

Problem

In the agent-platform Agent Builder UI, the "new agent" flow started with an automatic prompt: clicking the contextual header action (e.g. "New agent") opened the dock and immediately fired a canned prompt into the chat. Opening the sidebar shouldn't trigger a chat on its own — it should present an empty chat.

Changes

• The seeded prompt now prefills the composer instead of being auto-sent. The user reviews it (and can edit it) and hits send. This keeps the contextual header buttons as a one-click "auto prompt" affordance without starting a conversation by itself.
• Because prefilling is non-destructive, the start-fresh-vs-continue confirmation dialog is no longer needed and has been removed (AgentBuilderSeedDialog).
• The empty-state suggestion chips are unchanged — they still send on an explicit click.

How did you test this?

• pnpm --filter @posthog/ui typecheck — clean
• pnpm biome lint on the changed files — clean
• pnpm --filter @posthog/ui test — 1659 tests pass

Automatic notifications

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

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Created with PostHog from a Slack thread

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[github-actions]

React Doctor found 2 issues in 2 files · 2 warnings.

2 warnings

src/features/agent-applications/agent-builder/AgentBuilderDock.tsx

• ⚠️ src/features/agent-applications/agent-builder/AgentBuilderDock.tsx:338 Derived value copied into state no-derived-state

src/features/agent-applications/components/AgentChatSurface.tsx

• ⚠️ src/features/agent-applications/components/AgentChatSurface.tsx:132 Derived value copied into state no-derived-state

_{Reviewed by React Doctor for commit aa4d5e8.}