Structured error output with miette

[cdprice02]

CLAUDE.md mentions miette for user-facing errors with span context, but Cargo.toml doesn't include it yet.

Proposal: Integrate miette for parse errors and command failures so that users see clear, span-annotated diagnostics pointing to the exact part of their input that caused the error.

Example: An unclosed quote should highlight where the quote started.

This embodies ferrish's clarity over cleverness principle.

Acceptance criteria:

• miette added as a dependency
• Parse errors include source spans
• Error output is colorized and user-friendly
• Existing ShellError variants enhanced with span info where applicable