feat: implement BuildTimer v0.1.0 - #1

Summary

• add Linux-first Rust CLI for timing wrapped commands
• execute wrapped processes directly without shell interpolation
• preserve wrapped process exit codes
• store local-only sanitized history and compare against the previous matching command
• add history and clear commands
• handle command-start failures cleanly
• add unit/integration tests, README, CONTRIBUTING, SECURITY, and GitHub Actions CI

Privacy / security

• no telemetry, cloud, server, or account system
• no environment snapshot persistence
• redact environment-style assignments, common secret-bearing flags, authorization values, and URL credentials before history persistence

Required checks

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features

No release is created in this PR. Real-machine testing should happen before publishing v0.1.0.