Per-plugin token resolution chain

[ewega]

Summary

Implement per-plugin token resolution from environment variables and env files, so each plugin can have its own credential without conflicts.

Design

• GITHUB_TOKEN / GH_TOKEN -> GitHub and Copilot plugins
• GITLAB_TOKEN -> GitLab plugin
• AZURE_DEVOPS_PAT -> Azure DevOps plugin
• .devlake.env can contain multiple tokens with prefixed keys
• Token resolution chain per plugin: flag -> env file -> env var -> prompt

Changes

• Extend token.Resolve to accept plugin name and resolve from plugin-specific env vars
• Update .devlake.env format to support plugin-specific keys
• Each ConnectionDef specifies its env var names

Acceptance Criteria

• Each plugin resolves its own token independently
• .devlake.env supports GITHUB_TOKEN, GITLAB_TOKEN, etc.
• Prompt specifies which plugin's token is being requested
• Existing GITHUB_TOKEN / GH_TOKEN behavior unchanged