Add use_aws_platform flag for Claude Platform on AWS (parallel to use_bedrock/use_vertex/use_foundry)

## Background

Anthropic GA'd **Claude Platform on AWS** on 2026-05-11:
- Announcement: https://claude.com/blog/claude-platform-on-aws
- Docs: https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws

It's a fifth auth/transport surface alongside the four this action currently supports (direct Anthropic, Bedrock, Vertex, Foundry). Anthropic's official SDKs ship first-class support (`from anthropic import AnthropicAWS` in Python and equivalents in TS / Go / Java / C# / PHP / Ruby), and the Claude Code CLI itself reads `ANTHROPIC_AWS_WORKSPACE_ID` and `ANTHROPIC_AWS_BASE_URL` env vars natively.

## Current state

`action.yml` exposes `use_bedrock`, `use_vertex`, `use_foundry` and forwards the relevant env vars (`CLAUDE_CODE_USE_BEDROCK`, `AWS_REGION`, `ANTHROPIC_VERTEX_PROJECT_ID`, `ANTHROPIC_FOUNDRY_BASE_URL`, etc.) but **no** `use_aws_platform` flag and no `ANTHROPIC_AWS_WORKSPACE_ID` / `ANTHROPIC_AWS_BASE_URL` passthrough. `docs/cloud-providers.md` documents the four existing providers and doesn't reference Platform on AWS.

This means: setting only `ANTHROPIC_AWS_WORKSPACE_ID` on a step and omitting `anthropic_api_key` triggers the action's validation error ("must provide one of: api_key, use_bedrock, use_vertex, use_foundry"). There's no path through the action today.

## Proposed

Add the fifth provider in parallel to the existing four:

- New input: `use_aws_platform: \"true\"` (or whatever naming matches the public-facing name)
- Forward to the wrapped Claude Code CLI:
- `ANTHROPIC_AWS_WORKSPACE_ID` (from \${{ env.ANTHROPIC_AWS_WORKSPACE_ID }} — required when use_aws_platform is true)
- `ANTHROPIC_AWS_BASE_URL` (optional; SDK defaults to `https://gateway.{region}.api.aws` from `AWS_REGION`)
- `ANTHROPIC_AWS_API_KEY` (optional; SDK falls back to SigV4 via the AWS credential chain if absent)
- Reuse the existing OIDC pattern (`aws-actions/configure-aws-credentials@v4`) for credential supply
- Add a section to `docs/cloud-providers.md` matching the Bedrock/Vertex/Foundry templates

## Concrete use case

Mid-size org with ~\$1k+/month CI Claude spend wants to consolidate dev-side spend onto AWS commits via Platform-on-AWS. Direct Anthropic CI workflows (UX reviewer in our case) are the lowest-risk migration target — but the action's lack of a `use_aws_platform` flag blocks them today.

Happy to test a draft end-to-end against our PR reviewer workflow.

Start in action.yml by tracing the existing use_bedrock, use_vertex, and use_foundry inputs, validation, and environment-variable forwarding. Then read docs/cloud-providers.md and mirror the existing provider documentation; done means the new AWS Platform input and AWS environment variables pass through correctly and the validation accepts this authentication path.

Written by the indexing model from the issue text.