[02-use-cases] Add securing-coding-agents sample with Cedar policies

**Feature Request**

**Description**
Add a use case sample demonstrating how to secure AI coding agents with Cedar policies on AgentCore Gateway. This fills a gap in the existing samples — the current 02-policy feature tutorial focuses on ABAC with JWT claims for insurance underwriting, but there's no sample showing the coding agent security pattern (path-based file access, command blocklists, tool-level restrictions).

**Motivation**
Every team building AI coding assistants (with Strands, LangGraph, etc.) needs guardrails to prevent agents from:

Accessing files outside the workspace (/etc/passwd, SSH keys)
Running dangerous shell commands (rm -rf /, sudo)
Using unauthorized tools (code execution, HTTP, external APIs)
This is one of the most common questions from teams adopting AgentCore for coding agents.

**Proposed Solution**
A complete sample under 02-use-cases/securing-coding-agents/ that demonstrates:

Cedar policies using context.input conditions (path patterns, command strings)
Forbid-first patterns layered with explicit permit allowlists
Per-target Lambda functions (file, shell, restricted)
18 scenarios (8 ALLOW / 10 DENY) verifying policy enforcement
CloudWatch Logs integration for policy decision auditing
Security documentation for production adaptation
Strands agent demo via MCP

**Implementation**
Ready — PR will follow immediately after this issue is created.

Start by comparing the existing 02-policy feature tutorial with the proposed 02-use-cases/securing-coding-agents/ sample. Define the sample around AgentCore Gateway, Cedar policies, per-target Lambda functions, CloudWatch Logs, and the Strands MCP demo. Done means 18 scenarios pass with 8 ALLOW and 10 DENY decisions, with production security guidance included.

Written by the indexing model from the issue text.