feat: add AWS Bedrock AgentCore browser provider (native Rust) (#397)

* feat: add AWS Bedrock AgentCore browser provider (native Rust)

- Add agentcore provider with SigV4 authentication
- AWS SDK deps are optional behind 'agentcore' feature flag
- Build with: cargo build --features agentcore
- Supports AGENTCORE_REGION, AGENTCORE_PROFILE_ID, AGENTCORE_BROWSER_ID env vars
- Returns session ID and Live View URL in launch response
- Add connect_cdp_with_headers for signed WebSocket connections

* test: add unit tests for AgentCore provider

* refactor: use lightweight manual SigV4 signing instead of AWS SDK

- Replace aws-sigv4/aws-config with manual HMAC-SHA256 signing
- Removes ~60s compile time and significant binary size
- Credentials read from AWS_ACCESS_KEY_ID/AWS_SECRET_ACCESS_KEY env vars
- Supports AWS_SESSION_TOKEN for temporary credentials

* fix: correct AgentCore API endpoints

- Host: bedrock-agentcore.{region}.amazonaws.com
- Start session: PUT /browsers/{id}/sessions/start
- Stop session: PUT /browsers/{id}/sessions/stop
- Add urlencoding for browser ID in path
- Add AWS_DEFAULT_REGION fallback

* fix: use profileConfiguration.profileIdentifier for AgentCore profile

The AWS Bedrock AgentCore API expects profile configuration in the format:
{
  "profileConfiguration": {
    "profileIdentifier": "<profile-id>"
  }
}

Not the flat "profileId" field that was previously used.

* feat: support AWS credential provider chain via AWS CLI

- Try env vars first (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY)
- Fall back to 'aws configure export-credentials --format env'
- Honor AWS_PROFILE environment variable
- Works with SSO, IAM roles, credential files, etc.

---------

Co-authored-by: Chris Tate <chris@ctate.dev>

cli/src/output.rs

@@ -2808,7 +2808,7 @@ Options:
                              e.g., --proxy-bypass "localhost,*.internal.com"
   --ignore-https-errors      Ignore HTTPS certificate errors
   --allow-file-access        Allow file:// URLs to access local files (Chromium only)
-  -p, --provider <name>      Browser provider: ios, browserbase, kernel, browseruse, browserless
+  -p, --provider <name>      Browser provider: ios, browserbase, kernel, browseruse, browserless, agentcore
   --device <name>            iOS device name (e.g., "iPhone 15 Pro")
   --json                     JSON output
   --annotate                 Annotated screenshot with numbered labels and legend
@@ -2863,7 +2863,7 @@ Environment:
   AGENT_BROWSER_ANNOTATE         Annotated screenshot with numbered labels and legend
   AGENT_BROWSER_DEBUG            Debug output
   AGENT_BROWSER_IGNORE_HTTPS_ERRORS Ignore HTTPS certificate errors
-  AGENT_BROWSER_PROVIDER         Browser provider (ios, browserbase, kernel, browseruse, browserless)
+  AGENT_BROWSER_PROVIDER         Browser provider (ios, browserbase, kernel, browseruse, browserless, agentcore)
   AGENT_BROWSER_AUTO_CONNECT     Auto-discover and connect to running Chrome
   AGENT_BROWSER_ALLOW_FILE_ACCESS Allow file:// URLs to access local files
   AGENT_BROWSER_COLOR_SCHEME     Color scheme preference (dark, light, no-preference)