Set secure file permissions (0600) for config.yaml files

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Config files containing API keys and other sensitive data should not be world-readable. This PR changes the file creation mode to 0600 (owner read/write only) when creating or updating config.yaml files.

Changes:

• Modified getConfigYamlPath() in core/util/paths.ts to set mode 0600 when creating initial config.yaml
• Modified editConfigYaml() in core/util/paths.ts to set mode 0600 when updating config.yaml
• Modified config conversion command in extensions/vscode/src/commands.ts to set mode 0600

Security Impact:
This prevents accidentally exposing API keys and other credentials in config.yaml to other users on multi-user systems.

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Summary by cubic

Set config.yaml permissions to 0600 (owner read/write only) when creating or updating the file in both core and VS Code extension paths. This prevents world-readable configs and reduces the risk of leaking API keys on multi-user systems.

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