[cli] Add ans variable + variable assignment and evaluation

[forfudan]

This PR adds end-to-end variable support to the Decimo CLI calculator (tokenization → parsing → evaluation) and wires it into the interactive REPL with ans and name = expr assignment, plus corresponding tests and docs updates.

Changes:

• Introduce TOKEN_VARIABLE and extend the tokenizer/parser/evaluator pipeline to handle variable references.
• Add REPL variable store with ans tracking and name = expr assignment support (with reserved-name validation).
• Add tests for variable tokenization and evaluation; update planning/docs and decimo exports (Integer alias).

[Copilot]

Pull request overview

This PR adds end-to-end variable support to the Decimo CLI calculator (tokenization → parsing → evaluation) and wires it into the interactive REPL with ans and name = expr assignment, plus corresponding tests and docs updates.

Changes:

• Introduce TOKEN_VARIABLE and extend the tokenizer/parser/evaluator pipeline to handle variable references.
• Add REPL variable store with ans tracking and name = expr assignment support (with reserved-name validation).
• Add tests for variable tokenization and evaluation; update planning/docs and decimo exports (Integer alias).

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
tests/cli/test_tokenizer.mojo	Adds tokenizer tests for variable tokens and precedence vs funcs/constants.
tests/cli/test_evaluator.mojo	Adds evaluator tests for variable resolution (including ans) and undefined-variable errors.
src/decimo/prelude.mojo	Re-exports Integer from bigint in the prelude.
src/decimo/bigint/bigint.mojo	Adds comptime Integer = BigInt alias.
src/decimo/init.mojo	Exports Integer at the package root.
src/cli/calculator/tokenizer.mojo	Adds TOKEN_VARIABLE and optional known-variable handling during identifier lexing.
src/cli/calculator/repl.mojo	Implements REPL variable store, assignment parsing, reserved-name validation, and updates banner.
src/cli/calculator/parser.mojo	Treats TOKEN_VARIABLE like numbers/constants in shunting-yard output.
src/cli/calculator/evaluator.mojo	Resolves TOKEN_VARIABLE via a provided Dict[String, Decimal] and raises on undefined vars.
src/cli/calculator/engine.mojo	Threads variable names into tokenization and passes variable dict into evaluation; adds evaluate_and_return.
src/cli/calculator/init.mojo	Exports TOKEN_VARIABLE and evaluate_and_return.
docs/todo.md	Adds a TODO note about potentially flipping Decimal/BigDecimal naming.
docs/plans/cli_calculator.md	Marks Phase 4 variable features as complete and extends the plan table.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.