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/src/core/model-registry.ts

@@ -37,6 +37,7 @@ const ModelDefinitionSchema = Type.Object({
 			Type.Literal("openai-codex-responses"),
 			Type.Literal("anthropic-messages"),
 			Type.Literal("google-generative-ai"),
+			Type.Literal("bedrock-converse-stream"),
 		]),
 	),
 	reasoning: Type.Boolean(),
@@ -63,6 +64,7 @@ const ProviderConfigSchema = Type.Object({
 			Type.Literal("openai-codex-responses"),
 			Type.Literal("anthropic-messages"),
 			Type.Literal("google-generative-ai"),
+			Type.Literal("bedrock-converse-stream"),
 		]),
 	),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),