feat: add initial linter diagnostics integration path #104

Add the initial diagnostics/linter integration path for common languages without relying on VS Code extension runtime execution.

Scope:

• deterministic diagnostics model
• non-fatal missing linter handling
• project-local tool/config detection
• initial integrations through existing LinterBridge/service boundaries
• no auto-install of external tools
• no hidden command execution from extension metadata

Initial targets:

• Python via ruff when available
• JSON parser/schema validation path
• shellcheck/eslint/cargo checks only when explicitly available and configured

Done when:

• diagnostics can represent file, line, column, severity, source, and message
• missing tools do not crash ECLI
• tests cover diagnostic parsing and missing-tool behavior