Stand up behave.hs, a Gherkin runner for Haskell

## Summary

A BDD test runner that reads `.feature` files and binds Gherkin steps to Haskell functions — Python's `behave`, without the Python.

## Motivation

Haskell's test story is strong at the property and unit ends (`hspec`, `QuickCheck`, `tasty`) and absent at the acceptance end. A Haskell project that wants an executable specification a non-programmer can read has nothing current to reach for — the existing Cucumber ports are unmaintained or never landed. The step-definition-by-pattern model is the part worth borrowing; everything downstream of the parser can be Haskell-native.

## Scope

- Create the repo via the `scaffold-repo` flow.
- Suggested module inputs — `description`: "Run Gherkin .feature files against Haskell step definitions, under cabal test."; `topics`: `["haskell", "bdd", "gherkin", "cucumber", "testing"]`.
- Gherkin parser: Feature, Background, Scenario, Scenario Outline with Examples, Given/When/Then/And/But, tags, doc strings, data tables.
- Step registration binding a pattern to a handler, with captured arguments arriving typed rather than as `Text`.
- A context value threaded through the steps of a scenario.
- Hooks before and after feature, scenario, and step, scopeable by tag.
- Runner reporting pass / fail / undefined / skipped, printing a copy-pasteable stub for every undefined step.
- `hspec` or `tasty` integration so features run under `cabal test` alongside the existing suite.

## Open questions

1. **Step-argument typing.** Regex captures plus a `Parsable` class (behave and Cucumber's model), or a typed pattern DSL where an arity or type mismatch is a compile error? The second is the reason to do this in Haskell at all.
2. **Grammar fidelity.** Target the official Gherkin grammar so `.feature` files port between runners, or allow a Haskell-flavoured dialect?
3. **Context threading.** Explicit argument, `StateT`, or run steps in the user's own monad?
4. **Name.** `behave.hs` reads as a port and sets the expectation of behave-compatible step definitions, which isn't the plan.

## Done when

- [ ] Repo created at `alunduil/behave.hs` and adopted into Terraform (module block + `import`)
- [ ] Follow-up filed to drop the adoption `import` after apply
- [ ] Open questions above resolved and recorded in the new repo
- [ ] First milestone issue opened in the new repo