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.

PRINCIPLES.md

@@ -0,0 +1,67 @@
+# Principles
+
+Four principles governing engineering choices across all Corviduo
+projects. Common lodestar: **fitness-for-purpose**. Each principle
+rejects a different substitute goal that disguises itself as virtue.
+
+These are user-level defaults that originate at `~/.claude/CLAUDE.md`,
+duplicated here so they live with the codebase and are visible to
+humans browsing the repo (or AI tooling that doesn't load the
+user-level CLAUDE.md). When the user-level statement is amended,
+this file is backported.
+
+---
+
+## 1. Excellence over uniqueness
+
+Pick the shape that's right for the problem, not the shape that's new.
+If uniqueness is a byproduct of excellence, so be it; do not target it.
+
+*Rejects:* novelty-as-goal.
+
+## 2. Explicit over implicit
+
+Make load-bearing assumptions, constraints, and coupling visible.
+Idiomatic implicitness (language conventions, well-known protocols)
+is fine — the rule targets *invisible* implicitness, not all of it.
+Cost of explicitness is verbosity; cost of invisible implicitness is
+undetectable coupling. Pay the cost where auditability beats the tax.
+
+*Rejects:* cleverness-as-goal.
+
+## 3. Elegance is a byproduct, not a target
+
+Excellent engineering often produces elegant results; targeting
+elegance directly tends to produce cleverness, which is a different
+thing. Review test: *"fit-for-purpose, debuggable, consistent"* —
+not *"feels nice."*
+
+*Rejects:* beauty-as-goal.
+
+## 4. Action-relevance over thoroughness
+
+Before investigating, ask whether the answer would change the action.
+If both outcomes lead to the same default, skip the question.
+
+*Rejects:* thoroughness-as-goal.
+
+---
+
+## What this file is, and isn't
+
+These principles are *what to optimize for*. They sit above two other
+layers:
+
+| Layer | Where it lives | What it answers |
+|---|---|---|
+| **Principles** (this file) | `PRINCIPLES.md` + `~/.claude/CLAUDE.md` § Core principles | *What to optimize for* across all work |
+| **Behavioral guidelines** | `CLAUDE.md` § BEHAVIORAL GUIDELINES | *How to operate session-to-session* (Think Before Coding, Simplicity First, Surgical Changes, Goal-Driven Execution) |
+| **Workflow mechanics** | `CLAUDE.md` § Project conventions + the contract format, TDD cycle, etc. | *Project-level mechanics* (contracts-first, TDD vertical slices, no-backwards-compat-until-v1.x, persistent-memory commit-along) |
+
+Don't conflate the three layers. Workflow rules tell you what to do
+mechanically; behavioral guidelines tell you how to think during a
+session; principles tell you what *good* looks like across all of it.
+
+A workflow rule can be amended without touching the principle that
+motivates it. A principle changes rarely — when one does, it ripples
+through every other layer.