refactor(formula): extract formula parser into separate crate

Extract the formula AST and parser into a dedicated `improvise-formula`
crate and convert the project into a Cargo workspace.

The root crate now re-exports `improvise-formula` as `crate::formula` to
maintain backward compatibility for internal callers. The repository map is
updated to reflect the new crate structure.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

Cargo.lock

@@ -737,6 +737,7 @@ dependencies = [
  "dirs",
  "enum_dispatch",
  "flate2",
+ "improvise-formula",
  "indexmap",
  "pest",
  "pest_derive",
@@ -750,6 +751,14 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "improvise-formula"
+version = "0.1.0-rc2"
+dependencies = [
+ "anyhow",
+ "serde",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.14.0"