feat: support custom endpoint and bearer token configuration in TUI #675

Problem

Configuring a custom gateway endpoint (e.g. for Amazon Bedrock behind a corporate proxy) currently requires manually editing ~/.config/altimate-code/altimate-code.json and setting AWS_BEARER_TOKEN_BEDROCK as an environment variable or running providers login from the CLI.

This is friction for customers who expect to configure everything from within the TUI.

Proposal

Add TUI flows for:

1. Custom endpoint configuration — When selecting or configuring the amazon-bedrock provider, allow the user to enter a custom baseURL/endpoint directly in the TUI. This should write to the global config (provider.amazon-bedrock.options.baseURL).

2. Bearer token entry — Allow entering the AWS_BEARER_TOKEN_BEDROCK token from a TUI prompt (similar to how providers login works on the CLI, but accessible from within an active TUI session).

Context

Customers using custom LLM gateways on top of Bedrock (e.g. https://api.<URL>/claude-code-aws-bedrock/v1) currently need to:

• Manually create/edit altimate-code.json with the correct provider.amazon-bedrock.options.baseURL
• Set AWS_BEARER_TOKEN_BEDROCK in their shell profile or run altimate-code providers login -p amazon-bedrock separately

This is especially painful for first-time setup where the user hasn't created a config file yet.

Relevant Code

• Provider handler: packages/opencode/src/provider/provider.ts (line 301, "amazon-bedrock" handler)
• Config schema: packages/opencode/src/config/config.ts (line 1008, provider options)
• CLI login: packages/opencode/src/cli/cmd/providers.ts (line 250, ProvidersLoginCommand)
• Auth storage: packages/opencode/src/auth/service.ts