Remove skill buttons from the task header

[cvolzer3]

Summary

• remove the skill-action split button and dropdown from the task header
• delete skill-button prompt metadata, analytics, optimistic state, transcript presentation, stories, and visual baselines
• preserve the experiment-discovery prompt while rendering historical tagged prompts as ordinary user messages

Why

This removes the top-right shortcuts that automatically sent PostHog instrumentation skill prompts to the agent.

Impact

Users no longer see or invoke Track events, Track errors, Trace LLM calls, Capture logs, Add feature flag, or Run experiment from the task header. Existing local transcripts keep their underlying prompt text and render it as a standard user message.

Validation

• pnpm typecheck (commit hook)
• pnpm --filter @posthog/core test (2,808 tests)
• pnpm --filter @posthog/ui test (2,375 tests)
• targeted legacy/group tests (37 tests)
• pnpm --filter code build-storybook
• Biome check
• host boundary check

[trunk-io]

Merging to main in this repository is managed by Trunk.

• To merge this pull request, check the box to the left or comment /trunk merge below.

After your PR is submitted to the merge queue, this comment will be automatically updated with its status. If the PR fails, failure details will also be posted here

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 184e46d.}

[posthog]

👋 Visual changes detected for this PR.

Review and approve in PostHog Visual Review

If these changes are unexpected, they may be caused by a flaky test or a broken snapshot on master. Don't approve — rerun the job or wait for a fix.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "refactor(skill-buttons): remove header a..." | Re-trigger Greptile}