init: seed Ratatoskr from corviduo-project-template + ship v0 scaffold

Worldtree Conversation API debug TUI. Multi-pane observability dashboard:
chat transcript + persona/Vili affect log + tool events + admin events +
Bifrost state + tool inventory + (opt-in) raw server log.

Design locked at docs/design-brief.md (originated as
brokkr-smithy/docs/ratatoskr-design-brief.md). Operator-locked decisions:

- Textual application-shell framework (multi-pane dashboard, not REPL).
- Separate repo + separate dev team (no Worldtree-source imports).
- httpx-sse for SSE consumption (reference Python SSE-resume impl).
- Triple version-skew mitigation: spec-pin in pyproject.toml + recorded
  SSE snapshot tests + conformance smoke. Initial pin: Worldtree v0.19.0
  at 55101e909abcd2219833266b6f905c5bc956e0f0.
- Persona pane: label-don't-refuse PII posture.
- Server-log pane: opt-in via --server-log <path>.
- Two-stage Ctrl-C (cancel then exit).
- Markdown rendering default-on; --raw opt-out.

In the box:

- docs/design-brief.md — the locked design with full rationale.
- docs/SPEC-PIN.md — Worldtree spec pin + bump procedure.
- docs/conversation-api-spec.md + docs/conversation_api.contract.md —
  vendored Worldtree spec snapshots at the pinned SHA.
- pyproject.toml — Python 3.12, hatchling, uv-managed, deps locked.
- src/ratatoskr/ — stub package (cli.py raises NotImplementedError).
- tests/test_no_worldtree_imports.py — boundary smoke test PASSING.
- tests/snapshots/README.md — recording convention for SSE snapshot tests.

Not in the box yet:

- Gitea remote (operator/infra-ops to register at vh/ratatoskr).
- Implementation — the dev team owns this; design brief is the spec.

Origin: althing thread 01KS3R34XD3N6HMK91VXESHGW7 (worldtree-dev →
brokkr-smithy-dev, 2026-05-20). Volva consulted via thread
01KS3VF6W33N3V5FNMGQ91YNVD.

tests/test_no_worldtree_imports.py

@@ -0,0 +1,45 @@
+"""Boundary smoke test: Ratatoskr must not import from a Worldtree checkout.
+
+The boundary rule (docs/design-brief.md §2): Ratatoskr depends on the
+*published Conversation API spec* — vendored at `docs/conversation-api-spec.md`
+at a pinned SHA — and NOT on any Worldtree source code. This test fails
+loud if any file under `src/ratatoskr/` imports from `core.*`, `worldtree.*`,
+or any other identifiable Worldtree module path.
+
+If you find yourself wanting to import from Worldtree: stop. The right
+move is one of:
+  - Re-derive the type from the vendored spec.
+  - File a question to worldtree-dev via althing (the spec is unclear).
+  - Bump the spec pin (`docs/SPEC-PIN.md`) and re-vendor if the spec moved.
+"""
+from __future__ import annotations
+
+import pathlib
+import re
+
+_FORBIDDEN_IMPORT_PATTERNS = [
+    re.compile(r"^\s*import\s+core(\.|\s|$)", re.MULTILINE),
+    re.compile(r"^\s*from\s+core(\.|\s+import)", re.MULTILINE),
+    re.compile(r"^\s*import\s+worldtree(\.|\s|$)", re.MULTILINE),
+    re.compile(r"^\s*from\s+worldtree(\.|\s+import)", re.MULTILINE),
+]
+
+_SRC_ROOT = pathlib.Path(__file__).parent.parent / "src" / "ratatoskr"
+
+
+def test_no_worldtree_imports() -> None:
+    violations: list[tuple[pathlib.Path, str]] = []
+    for path in _SRC_ROOT.rglob("*.py"):
+        text = path.read_text(encoding="utf-8")
+        for pattern in _FORBIDDEN_IMPORT_PATTERNS:
+            for match in pattern.finditer(text):
+                line = text[: match.start()].count("\n") + 1
+                violations.append((path, f"line {line}: {match.group(0).strip()}"))
+    if violations:
+        report = "\n".join(f"  {p.relative_to(_SRC_ROOT.parent.parent)}: {v}" for p, v in violations)
+        raise AssertionError(
+            "Ratatoskr must not import from Worldtree source. Violations:\n"
+            f"{report}\n\n"
+            "See docs/design-brief.md §2 (boundary rule) and "
+            "docs/SPEC-PIN.md (spec-pin discipline)."
+        )