feat(ai): Add Amazon Bedrock provider (#494)

Adds support for Amazon Bedrock with Claude models including:
- Full streaming support via Converse API
- Reasoning/thinking support for Claude models
- Cross-region inference model ID handling
- Multiple AWS credential sources (profile, IAM keys, API keys)
- Image support in messages and tool results
- Unicode surrogate sanitization

Also adds 'Adding a New Provider' documentation to AGENTS.md and README.

Co-authored-by: nickchan2 <nickchan2@users.noreply.github.com>

packages/coding-agent/README.md

@@ -211,6 +211,29 @@ Credentials stored in `~/.pi/agent/auth.json`. Use `/logout` to clear.
 - **Token expired / refresh failed:** Run `/login` again for the provider to refresh credentials.
 - **Usage limits (429):** Wait for the reset window; pi will surface a friendly message with the approximate retry time.
 
+**Amazon Bedrock:**
+
+Amazon Bedrock supports multiple authentication methods:
+
+```bash
+# Option 1: AWS Profile (from ~/.aws/credentials)
+export AWS_PROFILE=your-profile-name
+
+# Option 2: IAM Access Keys
+export AWS_ACCESS_KEY_ID=AKIA...
+export AWS_SECRET_ACCESS_KEY=...
+
+# Option 3: Bedrock API Key (bearer token)
+export AWS_BEARER_TOKEN_BEDROCK=...
+
+# Optional: Set region (defaults to us-east-1)
+export AWS_REGION=us-east-1
+
+pi --provider amazon-bedrock --model global.anthropic.claude-sonnet-4-5-20250929-v1:0
+```
+
+See [Supported foundation models in Amazon Bedrock](https://docs.aws.amazon.com/bedrock/latest/userguide/models-supported.html).
+
 ### Quick Start
 
 ```bash