feat(core): add AWS profile setup for Bedrock

[rekram1-node]

/connect → Amazon Bedrock
  ├── Bedrock API key                       (existing key method, now labeled)
  └── AWS profile (SSO or named profile)    (new  type: "external")
        form: profile
          options: profiles found in ~/.aws/config + ~/.aws/credentials (names only)
          + type your own answer
        saves: { type: "external", methodID: "aws-profile", metadata: { profile } }

model request
  credential.metadata.profile ──────────────▶ Bedrock settings.profile
  external credential on Bedrock ───────────▶ auth: "sigv4"   (ambient bearer token can't take over)
  AWS default chain(profile, ignoreCache) ──▶ re-reads files, so `aws sso login` applies without restart