Add Ruff linter with CI integration

[jserv]

This integrates Ruff as static analysis linter alongside the existing black formatter. The pyproject.toml configures targeted rule categories (Pyflakes, pycodestyle, pyupgrade, flake8-bugbear, flake8-builtins, and select pylint rules) with per-file ignores for structural patterns like tkinter wildcard imports and module-level globals in TUI/GUI code.

Code fixes applied across all three source files:

• Convert 189 .format() calls to f-strings
• Replace EnvironmentError/IOError aliases with OSError (16 sites)
• Remove useless (object) inheritance from 5 classes
• Add 'from e' to 2 re-raised exceptions for proper chaining
• Eliminate mutable default arguments in cache functions by using module-level lists with .clear() on re-entry
• Simplify comparisons with 'in', max(), and direct function refs
• Modernize super() call and from-import style
• Remove dead variable assignment and update stale docstrings

Close #11

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Summary by cubic

Add Ruff linter with CI enforcement and project config. Cleaned up code to satisfy lint rules, fixed cached-node state leakage in TUI/GUI search, and corrected the option-env warning message arguments.

• Refactors

  • Converted .format() calls to f-strings.
  • Standardized exceptions (use OSError, add chaining, avoid bare except).
  • Modernized imports and super(); removed redundant “object” bases.
  • Simplified comparisons and control flow.

• Bug Fixes

  • Cleared cached node lists on TUI/GUI re-entry to prevent stale search results.
  • Fixed option-env warning to use the correct argument order.

^{Written for commit 460ab77. Summary will update on new commits.}