feat(COMPT-50): implement Zod-based env schema validation engine

[y-aithnini]

• Add defineConfig(schema) returning ConfigDefinition
• Add ConfigDefinition.parse() — validates process.env synchronously, returns frozen fully-typed config, throws ConfigValidationError on failure
• Add ConfigValidationError extending Error with fields: ZodIssue[]
• Add zod as peerDependency (^3 || ^4)
• Install zod as devDependency for local development
• Update src/index.ts to export defineConfig, ConfigDefinition, ConfigValidationError
• Add node to tsconfig types array for process.env access

Summary

• What does this PR change?

Why

• Why is this change needed?

Checklist

• Added/updated tests (if behavior changed)
• npm run lint passes
• npm run typecheck passes
• npm test passes
• npm run build passes
• Added a changeset (npx changeset) if this affects consumers

Notes

• Anything reviewers should pay attention to?

[Copilot]

Pull request overview

Implements the initial Zod-backed environment schema validation “engine” for @ciscode/config-kit, exposing a defineConfig() API plus a dedicated validation error type for startup-time env validation.

Changes:

• Added defineConfig() and ConfigDefinition.parse() to validate process.env via a consumer-provided Zod object schema.
• Added ConfigValidationError that carries ZodIssue[] and formats a multi-issue error message.
• Updated public exports and package/tooling configuration (Zod peer+dev dependency, Node types in TS config).

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tsconfig.json	Adds node types to support process.env typing.
src/index.ts	Updates the public API exports to expose the new validation engine + error.
src/define-config.ts	Introduces ConfigDefinition + defineConfig() with safeParse and frozen output.
src/errors/config-validation.error.ts	Adds a custom error type wrapping ZodIssue[] with a multi-line message.
package.json	Adds zod as a peer dependency (and dev dependency for local development).
package-lock.json	Locks the added zod dev dependency and updates lock metadata accordingly.