feat(playground): tailor empty-state claim to selected agent

[samuv]

Summary

• The playground empty-state heading was hardcoded to "Test & evaluate your MCP Servers", which only describes the ToolHive Assistant use case. With Skill Engineer and custom agents now selectable per thread, that single tagline misrepresents the experience.
• The heading and provider-config subtext now resolve from the selected agent via a small pure helper (renderer/src/features/chat/lib/empty-state-copy.ts):
  • ToolHive Assistant (default / fallback) → keeps the existing copy.
  • Skill Engineer → "Build & audit your Skills".
  • Custom agents → "Chat with {agent.name}".
• chat-interface.tsx resolves the thread's agent with the same useThreadAgentId + useAgents pattern already used by AgentSelector.

Test plan

• pnpm run test run renderer/src/features/chat/components/__tests__/chat-interface.test.tsx — 37/37 pass (3 new cases under empty state (no messages) › per-agent empty-state copy).
• pnpm run type-check — clean.
• eslint on the three changed files — clean.
• Manual smoke (Docker running): pnpm run start
  • Land on /playground with no threads → empty state shows "Test & evaluate your MCP Servers".
  • Switch to Skill Engineer via the agent selector → heading reads "Build & audit your Skills".
  • Create a custom agent (e.g. "Reviewer"), switch to it → heading reads "Chat with Reviewer".
  • With no provider configured, confirm the subtext under each heading reads naturally and "Configure your providers" still opens the settings dialog.
  • Send a message → empty state disappears, normal chat layout renders.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the playground chat empty state so its heading and provider-configuration subtext reflect the currently selected agent instead of always using ToolHive Assistant copy.

Changes:

• Added a pure helper for resolving empty-state copy from an agent.
• Wired ChatInterface to load the thread’s selected agent and render agent-specific copy.
• Added component tests for default, Skill Engineer, and custom-agent empty-state copy.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
renderer/src/features/chat/lib/empty-state-copy.ts	Adds centralized empty-state copy resolution for built-in and custom agents.
renderer/src/features/chat/components/chat-interface.tsx	Uses agent hooks to select and render the appropriate empty-state heading/subtext.
renderer/src/features/chat/components/__tests__/chat-interface.test.tsx	Adds mocked agent data and tests for per-agent empty-state behavior.