Consolidate local credential configuration in config.toml

[oshorefueled]

Summary

Remove automatic credential loading from project-level .env and .env.local files. Use ~/.vectorlint/config.toml as the single persisted configuration file for local VectorLint credentials and provider settings.

Motivation

VectorLint currently supports the same provider configuration through both ~/.vectorlint/config.toml and project-level environment files. The overlapping paths increase setup complexity, make precedence harder to explain, and create documentation drift between workflows.

A single local configuration source makes setup and troubleshooting clearer.

Proposed change

• Stop reading .env and .env.local from the current project directory.
• Store local provider selection, credentials, and related VectorLint settings in ~/.vectorlint/config.toml.
• Preserve process environment variables for CI and externally injected secrets, but do not load them from local environment files.
• Remove .env setup and precedence guidance from user-facing documentation.
• Add an appropriate changelog or migration note for users currently relying on project .env files.

Acceptance criteria

• VectorLint no longer loads .env or .env.local automatically.
• ~/.vectorlint/config.toml is the only persisted local credential configuration source documented for users.
• Process environment variables still support CI and secret injection.
• Tests cover configuration precedence after local environment-file loading is removed.
• README, configuration, provider, quickstart, troubleshooting, and environment-variable documentation describe the consolidated workflow consistently.

Docs Affected

• README.md: Presents ~/.vectorlint/config.toml and a local .env file as credential and observability configuration options.
• CONFIGURATION.md: Describes project-scoped .env configuration as taking precedence over ~/.vectorlint/config.toml.
• docs/quickstart.mdx: Presents a project .env file as an alternative provider configuration source with precedence over config.toml.
• docs/llm-providers.mdx: Contains provider-specific .env examples, configuration precedence, and .gitignore guidance for local credential files.
• docs/env-variables.mdx: Lists project .env files as a configuration source and places them above global config.toml in the precedence order.
• docs/cli-reference.mdx: States that VectorLint reads project-level .env values and gives them precedence over global configuration.
• docs/troubleshooting.mdx: Recommends either a project .env file or ~/.vectorlint/config.toml when provider credentials are missing.
• docs/false-positive-tuning.mdx: Documents project .env files for local and CI-specific CONFIDENCE_THRESHOLD overrides.
• docs/configuration-schema.mdx: Presents .env files and process environment variables as locations for configurable values.