[Bug] Shell Configuration Not Validated - Invalid Config Accepted #375

Description

Shell configuration files are not validated before loading. Invalid syntax or malformed configurations are accepted and silently cause problems. No validation prevents broken configurations from being loaded.

Steps to Reproduce

1. User creates .devshell.toml with typo: depend[encies] instead of dependencies
2. Configuration loaded without validation
3. Feature silently breaks or behaves unexpectedly
4. Difficult for user to debug the issue

Environment Information

• Config format: TOML or JSON
• Validation: Schema not enforced
• Loading: Raw parsing without validation
• Error feedback: None

Expected Behavior

• Configuration validated against schema
• Clear error on invalid configuration
• Helpful error messages with hints
• Setup prevents invalid config

Actual Behavior

• No schema validation
• Invalid configs accepted
• Features silently break
• No error message

Code Reference

• File: src/devshell/config_parser.py
• Missing: Schema validation
• Missing: Error handling
• Missing: Validation messages

Additional Context

Level 2 usability issue. Fix requires:

• Implementing config schema validation
• Using jsonschema or similar validator
• Providing detailed error messages
• Testing with invalid configs

Suggested Labels

bug, configuration, validation, error-handling, ux