feat: add Claude Platform on AWS provider (anthropic-aws)

[Insung Park (magarage)]

Summary

Adds a dedicated anthropic-aws provider for Claude Platform on AWS — the Anthropic-managed offering fronted by AWS (API-key auth, AWS Marketplace billing).

The existing anthropic provider cannot target this endpoint: Claude Platform on AWS requires every request to carry an anthropic-workspace-id header identifying the workspace, and the current ChatAnthropic setup never sends it. Pointing ANTHROPIC_BASE_URL at the AWS gateway alone gets rejected. This PR wires that header in via a separate provider so the first-party anthropic provider is untouched.

What it does

• New provider anthropic-aws (selectable in --init, or auto-detected from ANTHROPIC_AWS_API_KEY), separate from the first-party anthropic provider.
• Env names mirror the official Anthropic AWS SDK/CLI:
  • ANTHROPIC_AWS_API_KEY → sent as x-api-key
  • ANTHROPIC_AWS_WORKSPACE_ID (wrkspc_…) → required, sent as the anthropic-workspace-id header
  • AWS_REGION (falls back to AWS_DEFAULT_REGION) → base URL is derived as https://aws-external-anthropic.{region}.api.aws
  • ANTHROPIC_AWS_BASE_URL → optional explicit override (proxy/testing)
• Startup validation with clear errors when the workspace ID or region/base URL is missing.
• --init wizard: adds region + workspace-ID steps (skipped when already resolvable); shows them in the setup checklist.
• Diagnostics: ANTHROPIC_AWS_API_KEY is redacted like other secrets; workspace ID / region / base URL are shown verbatim (not secrets).
• README documents the provider and setup.

Example

OPENWIKI_PROVIDER=anthropic-aws
ANTHROPIC_AWS_API_KEY=<your-key>
ANTHROPIC_AWS_WORKSPACE_ID=wrkspc_XXXXXXXXXXXXXX
AWS_REGION=us-west-2
# optional override:
# ANTHROPIC_AWS_BASE_URL=https://proxy.example/anthropic