feat(coding-dev): require domain values to be typed, not primitives

A value with rules of its own — a URL, a path, a closed set, an identifier —
gets a type of its own. Passing it as a bare string discards those rules at the
first function boundary and leaves every consumer re-deriving them.

Added as a subsection of Architecture Preferences, with a table mapping the
common cases to the type to reach for in any language, the reasoning, and the
boundary rule: parse once at the edge, keep the type inward, serialize back only
where the wire actually demands a string. Notes the counter-case, so this is not
read as a mandate to wrap every string.

python.md gains the concrete choices: yarl.URL (already present wherever aiohttp
is), pathlib, StrEnum, and NewType for non-interchangeable identifiers.

code-languages/python.md

@@ -48,6 +48,23 @@ disallow_untyped_defs = true
 - Third-party library missing stubs: install `types-*` if it exists, otherwise a narrowly
   scoped `# type: ignore[import-untyped]` with a comment — never a blanket module-level ignore.
 
+### Value types over primitives
+
+The general rule is in [`coding-dev.md` §7](skills-public/coding-dev.md); Python's concrete
+choices are:
+
+- **`yarl.URL` for URLs**, not f-string assembly. It ships with `aiohttp`, so any async HTTP
+  project already has it. `URL(base) / "api" / "v4" / "users"` joins without double slashes,
+  and `.with_query({...})` escapes parameters that string formatting would corrupt.
+- **`pathlib.Path` for paths**, never `os.path` + `str`. Ruff's `PTH` rules enforce this;
+  keep `os.fdopen` where a descriptor rather than a name is being adopted.
+- `enum.StrEnum` (3.11+) where a closed set must also serialize as a string, so the wire
+  format stays a string without the code comparing bare literals.
+- `typing.NewType` for identifiers that should not be interchangeable — a `UserId` and a
+  `RepoId` are both `int` and nothing otherwise stops them being swapped.
+- Parse at the boundary: take `str` in the public signature if callers supply one, convert
+  immediately, and keep the real type internal.
+
 ---
 
 ## 3. Linting