Add mcp init command to configure MCP clients

[gtsiolis]

Summary

Adds lstk mcp init — configures the user's installed MCP clients (Cursor, Claude Code, Claude Desktop, VS Code, Codex) to launch the LocalStack MCP server, so coding agents can drive LocalStack with one command.

This is a native Go reimplementation of the standalone setup wizard in localstack/localstack-mcp-server (PR #40), not a wrapper around npx … init. Rationale: lstk is a self-contained Go binary that has never required Node, and the users most likely to run this (Homebrew / raw-binary installs) often have no Node; reimplementing natively keeps the binary self-contained, reuses the auth token lstk already resolves (no token prompt), and matches lstk's output/sink house style.

What's included

• lstk mcp namespace + init verb.
• Docker-default server config so no Node is needed; --method npx switches to the host-Node launcher.
• Adapters in internal/mcpconfig/: file-based JSON (Cursor, Claude Desktop, VS Code — VS Code uses the divergent servers + type: stdio schema) and CLI-managed shell-out (Claude Code, Codex via their own mcp add).
• Flags: --method, --client, --token, --image-tag, --cache-dir, --workspace, --config KEY=VALUE. Auto-configures all detected clients by default.
• Entry kept byte-compatible with the npm wizard (key localstack, same specs) so the two installers are interchangeable.
• Secret-safe: token written 0600 and redacted from error output; Docker socket grant disclosed in output + help.

Follow-ups

• lstk mcp remove and lstk mcp status.
• Potentially convert the MCP server itself to Go as lstk mcp serve — first-party and engineering-owned, replacing the TypeScript @localstack/localstack-mcp-server. init would then default to --method lstk, pointing clients at lstk itself.

Closes PRO-317.