Add docs, Makefile, tests, CI, and feature plan

[sa-wright]

Summary

Iterates on documentation, adds a Makefile, introduces tests and CI, and adds a feature plan in case you're bored.

Documentation

• README: Prerequisites, quick start, language basics, shell/SSH/parallel-for examples with snippets, builtin reference (core, string/number methods, net), REPL, and project layout.
• Makefile: build, release, test, run SCRIPT=, repl, install, check (clippy + fmt), fmt, clean, help.
• docs/feature-plan.md: Prioritized feature plan (file I/O, argv/env, JSON, shell/SSH upgrades, REPL, etc.) and suggested implementation order.

Tests

• Lib split: lib.rs exposes run_script() and run_script_capture(); main.rs delegates to the lib.
• Unit tests: Lexer (scan_), parser (parse_), runtime/value (is_truthy, as_string, parse_duration, array/tuple).
• E2E tests: tests/e2e.rs — log, arithmetic, sort, find, functions, interpolated strings, parse/runtime error handling.
• Parser test updated for log 42 → Stmt::Invoke (not Expr).

CI

• GitHub Actions: Jobs for cargo test, cargo clippy, cargo fmt --all -- --check on push/PR to main/master.
• Runs in ubuntu:22.04; installs curl, build-essential, rustup (no composite actions or cache for act compatibility).
• Cargo commands source $HOME/.cargo/env. Clippy runs without -D warnings so CI passes while lints are addressed over time.
• Use act --container-architecture linux/amd64 for local CI runs.

Other

• .gitignore: .DS_Store, .idea/, .vscode/.
• Format: cargo fmt applied across the codebase.