fix: resolve CI/CD issues - remove unused dependencies and imports

- Remove unused thiserror dependency from Cargo.toml
- Remove unused imports (Text, Tabs, Widget, Event, KeyCode, KeyEventKind) from tui/mod.rs
- Remove unused imports (File, Write) from export/mod.rs
- Remove unused pub use ComplexityDistribution from core/analyzer.rs

techdebt-tracker-cli/rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "stable"
+components = ["rustfmt", "clippy"]