feat(mvp): working secrets-engine CLI for the whynot-design npm publish lane

Implements SECRETS-WP-0002 end to end as a uv-managed Python package:

- catalog: non-secret lane registry + strict validator (build/test/prod)
- stage roles + OpenBao ACL policies; guards refuse wildcards, sys/, identity/,
  admin names, and cross-stage paths before any backend call
- plan/apply: dry-run-first, idempotent policy + approle apply, decision-gated
- decisions: State Hub lookup with local-fixture fallback; non-secret evidence
  to JSONL + hub progress, scrubbed of any value
- provision/verify: mode-0600 file import + generated test values; positive/
  negative checks that never print the value
- exec delivery: `exec --catalog ... -- npm publish` injects the token via a
  temp .npmrc for the child only, cleaned up on exit/failure/interrupt
- ops-warden routing contract + hardening backlog docs
- 34 tests incl. live OpenBao integration; scripts/demo-e2e.sh runs the full
  chain against a throwaway bao dev server

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/secrets_engine/errors.py

@@ -0,0 +1,54 @@
+"""Typed errors with stable exit codes for the CLI.
+
+Exit codes are part of the contract so callers (ops-warden, CI) can branch on
+outcome without parsing text.
+"""
+from __future__ import annotations
+
+
+class SecretsEngineError(Exception):
+    """Base class. ``exit_code`` is the process exit status."""
+
+    exit_code = 1
+
+
+class CatalogError(SecretsEngineError):
+    """Catalog file missing, unparseable, or schema-invalid."""
+
+    exit_code = 2
+
+
+class DecisionError(SecretsEngineError):
+    """Decision/CCR missing, denied, superseded, stale, or unapproved."""
+
+    exit_code = 3
+
+
+class PolicyGuardError(SecretsEngineError):
+    """A plan violates a safety guard (wildcard, out-of-stage path, root, ...)."""
+
+    exit_code = 4
+
+
+class BackendError(SecretsEngineError):
+    """OpenBao backend call failed or is unreachable."""
+
+    exit_code = 5
+
+
+class ProvisioningError(SecretsEngineError):
+    """Provisioning input invalid (bad file mode, inside repo, missing field)."""
+
+    exit_code = 6
+
+
+class VerificationError(SecretsEngineError):
+    """A verification check did not produce the expected result."""
+
+    exit_code = 7
+
+
+class DeliveryError(SecretsEngineError):
+    """Exec-time delivery could not be set up safely."""
+
+    exit_code = 8