New sample: OpenCode on AgentCore (end-to-end, 5 capabilities)

Proposing a new end-to-end sample under `02-use-cases/opencode-on-agentcore/`.

An async coding-agent that demonstrates Runtime, Gateway, Identity, Policy, and Observability through a single deployable workload. FastMCP server with 6 MCP tools running in Firecracker microVMs, deployed via 9 CDK stacks. Tested in us-east-1 and eu-central-1.

PR ready at # (or "PR to follow").

Start by reviewing the existing samples under 02-use-cases/ and then scope the new opencode-on-agentcore/ directory around its nine CDK stacks. Verify that the deployable workload demonstrates Runtime, Gateway, Identity, Policy, and Observability with six MCP tools, and confirm deployment in us-east-1 and eu-central-1.

Written by the indexing model from the issue text.