fix(config): detect placeholder API keys as missing

[Redestiny]

Summary

• Align placeholder API key detection across doctor, init, and config loading so example values like your-api-key-here and sk-your-key-here are treated as missing config.
• Sync .env.example with currently supported optional env settings to reduce drift between docs and actual behavior.
• Add regression tests covering placeholder API keys and the generated .env template from init.

Type of change

• Bug fix
• New feature
• Refactor (no behavior change)
• Docs / SKILL.md
• Test
• Performance

Motivation (optional)

• Example API key placeholders could be misread by the CLI as real configuration, which makes doctor output and first-time setup guidance misleading.
• This change keeps the example files, validation logic, and tests aligned.

Changes

File	Change
.env.example	Normalize the API key placeholder
packages/core/src/utils/config-loader.ts	Add shared placeholder API key detection and treat placeholder values as missing
packages/core/src/index.ts	Export the shared API key detection helper for CLI reuse
packages/cli/src/utils.ts	Add a helper to detect configured API keys from .env file contents
packages/cli/src/commands/init.ts	Reuse the shared detection logic and clean up duplicated / missing .env example lines
packages/cli/src/commands/doctor.ts	Stop treating placeholder API keys as valid config
packages/core/src/__tests__/config-loader.test.ts	Add regression coverage for placeholder API keys
packages/cli/src/__tests__/cli-integration.test.ts	Add regression coverage for init output and doctor behavior

Testing

• pnpm test