feat: add Lightpanda browser agent engine definition and smoke test

[Copilot]

Adds a declarative engine definition for the Lightpanda headless browser agent — a Zig-based browser built for AI automation — along with a smoke test workflow.

shared/lightpanda.md

• engine.id: lightpanda, experimental: true, mcp: false (browser agent; no MCP client)
• Harness downloads the nightly lightpanda-x86_64-linux binary at runtime; no pre-install step
• Resolves LLM endpoint via /reflect, then runs:

  lightpanda agent --provider openai --model <model> --base-url <endpoint> --task <prompt>
  

• Strips provider prefix from model name (copilot/claude-sonnet-4-5 → claude-sonnet-4-5)
• Disables telemetry and core dumps; cleans up temp dir on exit

smoke-lightpanda.md

• Triggers on /smoke-lightpanda, workflow_dispatch, water label, or 🚀 reaction
• Asks the agent to navigate https://example.com and extract the page title
• Validation is exit-code only — Lightpanda's agent mode is browser-only and cannot write to $GH_AW_SAFE_OUTPUTS

Usage

engine:
  id: lightpanda
model: copilot/claude-sonnet-4-5
imports:
  - shared/lightpanda.md