feat: add GitHub Action for automated PR review via AgentCore Harness

Adds a workflow that triggers on PR open/reopen, checks team membership
in agentcore-cli-devs, and invokes the Bedrock AgentCore Harness to
review the PR and post comments. Also supports manual workflow_dispatch.

Includes the local service model since InvokeHarness is not yet in
standard boto3.

Constraint: InvokeHarness not in standard boto3 — local service model required
Constraint: Team membership check requires org-level read permissions on GITHUB_TOKEN
Confidence: high
Scope-risk: narrow

Author: jesseturner21