fix: add marketplace.json so /plugin marketplace add works

[tiffanylphan]

Summary

Adds .claude-plugin/marketplace.json so the repo can be registered as a Claude Code plugin marketplace.

Currently, following the README instructions to install via /plugin install fails because Claude Code's /plugin marketplace add launchdarkly/ai-tooling requires a marketplace.json — having only plugin.json produces:

Error: Marketplace file not found at /Users/<user>/.claude/plugins/marketplaces/launchdarkly-ai-tooling/.claude-plugin/marketplace.json

This PR is the minimal fix to unblock that flow:

• .claude-plugin/marketplace.json — single-plugin marketplace with "source": ".", pointing at the plugin defined in .claude-plugin/plugin.json at the repo root.
• README.md — updates the install steps to the commands that actually work (/plugin marketplace add then /plugin install), instead of "search for LaunchDarkly" (which only finds plugins in marketplaces already added — not this one).

Things to flag for reviewers

1. Plugin name collision with launchdarkly-labs/agent-skills. That marketplace also publishes a plugin called launchdarkly. A user who adds both marketplaces will have two plugins with the same name. If ai-tooling is meant to supersede the labs repo, that's fine — but if they're meant to coexist, one should probably rename. Worth deciding before this lands.
2. Cursor install instructions in the README are unchanged. I only fixed the Claude Code section because that's what I was debugging. The Cursor install steps ("Search for LaunchDarkly in the marketplace, or install from the repo URL") may have the same "search only works after it's published to a known marketplace" issue — someone who knows Cursor's plugin model should sanity-check.
3. Not yet listed in anthropics/claude-plugins-official. Until/unless it's submitted there, /plugin install browse without first running /plugin marketplace add will never find it. The updated README sidesteps this by telling users to add the marketplace explicitly.

Test plan

• git pull this branch locally
• In Claude Code: /plugin marketplace add launchdarkly/ai-tooling — should succeed
• /plugin install launchdarkly@launchdarkly-ai-tooling — should list and install the plugin
• Confirm launchdarkly:* skills become available (e.g., /launchdarkly:flag-cleanup)
• Confirm the LD MCP server prompts for auth on first use

🤖 Generated with Claude Code