feat(design-systems): import a design system out of a rulebook's prose

Milestone #254 step 3 (#2288). Reuses #251's prose extractor as the reader and
adds the part that makes it an import rather than a list of claims.

**The join is the whole trick.** A rulebook states a design system in two places
and neither half is a token: one rule names the colours ("Obsidian #14171A (page
bg, deepest surface)"), another names the custom properties
(`--fs-obsidian/iron/slate`). The import pairs them on the word — `--fs-obsidian`
ends with `obsidian` — which is the only reason it produces something usable
instead of seventy empty names. The parenthetical becomes the token's purpose,
which is the field a bare hex could never carry.

**Prohibitions arrive as replacements, per the operator's reframe.** Rule 52
declares Parchment and forbids pure white in one breath, so the import emits
"write --fs-parchment instead of #ffffff" — the same fact stated forwards. It
attaches to the FIRST token that rule supplied a value for, not to every token
of that rule, because claiming Vellum is also the replacement for white would be
putting words in the rulebook's mouth.

**A token the rulebook names but states no readable value for is still
proposed, with an empty value.** Radius steps and type sizes are prose ("Small
4px") and nothing here parses them; inventing a parse per shape would be
guessing. The name is real and the value needs a human, so the proposal says
exactly that — and the UI leads with the COUNT of those, because an import that
hid them would look more complete than it is.

Preview is the default on both surfaces and in the UI. An import is a proposal:
rulebooks are written aspirationally and some of what they describe was never
built, so every entry carries the rule id and the sentence it came from and a
reviewer can check the claim rather than trust it.

Existing token names are never overwritten. A value already in the record was
put there deliberately — most likely correcting this importer — so a re-run
fills gaps and lists the rest as skipped, which also makes it safe to repeat.

Colours the rulebook names but never exposes as a custom property produce no
token: it never asked for one, and inventing a name would put something in the
record no rule sanctions.

tests/test_routes_design_systems.py

@@ -24,8 +24,8 @@ def test_route_handlers_callable():
     for name in (
         "list_design_systems", "create_design_system", "get_design_system",
         "update_design_system", "delete_design_system", "resolve_design_system",
-        "list_design_tokens", "create_design_token", "update_design_token",
-        "delete_design_token", "set_project_design_system",
+        "import_design_system", "list_design_tokens", "create_design_token",
+        "update_design_token", "delete_design_token", "set_project_design_system",
     ):
         assert callable(getattr(routes, name))
 
@@ -44,6 +44,7 @@ def test_every_endpoint_is_reachable_on_the_app():
         "/api/design-systems",
         "/api/design-systems/<int:design_system_id>",
         "/api/design-systems/<int:design_system_id>/resolved",
+        "/api/design-systems/<int:design_system_id>/import",
         "/api/design-systems/<int:design_system_id>/tokens",
         "/api/design-tokens/<int:token_id>",
         "/api/projects/<int:project_id>/design-system",
@@ -57,7 +58,7 @@ def test_service_functions_take_user_id():
         "create_design_system", "list_design_systems", "get_design_system",
         "update_design_system", "delete_design_system", "resolve_design_system",
         "create_token", "list_tokens", "update_token", "delete_token",
-        "set_project_design_system",
+        "set_project_design_system", "import_from_rulebook",
     ):
         fn = getattr(svc, fn_name)
         assert callable(fn)
@@ -84,6 +85,12 @@ def test_agent_and_web_surfaces_stay_at_parity():
         assert callable(getattr(tools, name)), f"MCP tool missing: {name}"
         assert callable(getattr(routes, name)), f"REST route missing: {name}"
 
+    # Import is the one verb whose handler names differ between the surfaces
+    # (the tool says what it reads FROM; the route is already under the system),
+    # so the loop above can't pair it. It still has to exist on both.
+    assert callable(tools.import_design_system_from_rulebook)
+    assert callable(routes.import_design_system)
+
 
 def test_every_mcp_tool_in_the_module_is_registered():
     """A tool written but never registered is invisible to an agent, and nothing