G0: cell substrate — invariants made executable

Strand G0 complete and VERIFIED against real Postgres, not asserted.

  - FastAPI plane, fail-closed provider seams, repair-pointer error taxonomy
  - migration 001: all 10 tables incl. repo_type NOT NULL and model_cards (I-7)
  - 17 invariant tests, ruff clean, vocabulary audit clean

Two bugs found by RUNNING it that review would not have caught:

  1. SQLAlchemy Enum persists .name, not .value — so RepoState.deleted_soft
     and CreatedVia.imported would have written labels migration 001 never
     declared, failing at runtime rather than at review. Pinned via
     values_callable.
  2. op.create_table asks each Enum to emit its own CREATE TYPE with no
     checkfirst, so the second reference raised DuplicateObject and the
     migration died halfway. Types are now created once, referenced with
     create_type=False.

Proven live, with the hostile env var set:
  - I-12: COMMIT_SHA=deadbeef... in the environment, /version reports real HEAD.
    That env pin is the documented root cause of nine sibling services
    misreporting their commit; here it is structurally ignored.
  - I-8: three unconfigured providers -> status degraded, HTTP 503, each saying
    'refusing to report healthy'. No mock, no false green.
  - G0.4: upgrade -> downgrade -> upgrade round-trip clean (10 -> 0 -> 10).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

api/app/errors.py

@@ -0,0 +1,108 @@
+"""Repair-pointer error taxonomy (section 0.6, G8.3).
+
+EVERY error this service emits is a 4-field repair pointer:
+
+    {code, speak, machine_cause, remediation_tool}
+
+No exceptions, including validation errors. `speak` is grandma-words (I-9) and
+obeys the D-9 vocabulary law (see scripts/vocab_audit.py), and never uses the
+word "commit" on a shelter surface.
+`remediation_tool` names the tool an agent should call next, or null when a human
+must act.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import HTTPException
+
+
+class RepairPointer(HTTPException):
+    """An error an agent can act on without guessing."""
+
+    def __init__(
+        self,
+        status_code: int,
+        code: str,
+        speak: str,
+        machine_cause: str,
+        remediation_tool: str | None = None,
+        **extra: Any,
+    ) -> None:
+        self.code = code
+        self.speak = speak
+        self.machine_cause = machine_cause
+        self.remediation_tool = remediation_tool
+        super().__init__(
+            status_code=status_code,
+            detail={
+                "code": code,
+                "speak": speak,
+                "machine_cause": machine_cause,
+                "remediation_tool": remediation_tool,
+                **extra,
+            },
+        )
+
+
+def provider_unconfigured(provider: str, missing: str) -> RepairPointer:
+    """I-8: fail closed. Never answer from a mock, never claim live.
+
+    This is the guard the domains cell did not have on its public quote route.
+    """
+    return RepairPointer(
+        status_code=503,
+        code="provider_unconfigured",
+        speak=(
+            "That part of Windy Git isn't switched on yet, so we're not going to "
+            "guess at an answer. Nothing you have is affected."
+        ),
+        machine_cause=f"{provider} is not configured: {missing} is unset",
+        remediation_tool=None,
+        provider=provider,
+    )
+
+
+def passport_unresolvable(passport: str, upstream_status: int) -> RepairPointer:
+    """G3.6: 404 and 400 REFUSE. There is no soft-allow path here.
+
+    windy-chat maps 400/429 to "unreachable" and then soft-ALLOWS, which is a
+    live residual bypass because 429 is trivially inducible at 100/min/IP.
+    """
+    return RepairPointer(
+        status_code=403,
+        code="passport_unresolvable",
+        speak="We couldn't confirm that helper's ID, so we didn't let it make changes.",
+        machine_cause=(
+            f"eternitas trust lookup for {passport} returned {upstream_status}; "
+            "policy is REFUSE on 400/404 and REFUSE after backoff on 429/5xx"
+        ),
+        remediation_tool="windy_git.reissue_agent_token",
+        passport=passport,
+    )
+
+
+def quota_exceeded(repo_id: str, used: int, limit: int) -> RepairPointer:
+    """G4.6: we emit the cross-sell hook; the KERNEL owns the price (I-11)."""
+    return RepairPointer(
+        status_code=413,
+        code="quota_exceeded",
+        speak=(
+            "Your projects have outgrown the space on your plan. Nothing was lost — "
+            "the newest save just didn't go through."
+        ),
+        machine_cause=f"repo {repo_id} would use {used} bytes against a limit of {limit}",
+        remediation_tool="windy_cloud.upgrade_storage",
+        repo_id=repo_id,
+    )
+
+
+def kit_zero_refused(host: str) -> RuntimeError:
+    """D-4 / section 7.8: only Grant may overturn a never."""
+    return RuntimeError(
+        f"REFUSING TO START: this service is pointed at Kit 0 ({host}). "
+        "Windy Git never runs on Kit 0 — CI executes untrusted code and Kit 0 "
+        "holds identity, the certificate authority, mail, Matrix and the broker. "
+        "See DNA_STRAND_MASTER_PLAN.md D-4."
+    )