phase 1: Go foundation — module, migrations, domain, seed ingest

Core deliverables:
- Go module github.com/dtoro/oikos (Go 1.26.3)
- cmd/oikos: single binary with role subcommands (migrate, seed, export)
- 6 SQL migrations: ontology meta-schema, entity instances (UUID+slug,
  blast_radius recursive function), operations (signals/checks/approvals),
  cognition (classifications/executions/feedback/patterns/skills), policy,
  observability (TimescaleDB hypertables + CAGGs + retention)
- Domain layer: entity, signal, execution, classification, pattern, skill,
  approval, check types + 11 sentinel errors + lifecycle state machines
- DB layer: pgx pool, SQL splitter (handles 94436 and -- comments), migration
  runner, seed ingest (ontology+inventory+policy) with content-hash dedup
- Config: env-based with defaults, secrets redaction
- Observability: slog JSON logger with debug mode
- Infrastructure: Makefile, docker-compose.yml, multi-stage Dockerfile
  (distroless, CGO_ENABLED=0)

Verified end-to-end against timescale/timescaledb:2.17.2-pg16:
- 6 migrations applied (65 SQL statements)
- Seeds ingested: 6 lifecycles, 59 entity types, 46 relationship types,
  111 entities, 144 relationships, 4 risk classes, 27 approval rules,
  9 autonomy settings
- Idempotent: second seed run is a no-op (content hash matches)

Bugs fixed during implementation:
- TimescaleDB CAGGs can't run in a transaction -> splitSQL() executes
  statements individually
- Semicolons in -- comments treated as separators -> comment handling
- YAML keys source/target didn't match code's source_type/target_type
- yaml.Marshal produced YAML for JSONB columns -> json.Marshal

internal/domain/errors.go

@@ -0,0 +1,21 @@
+package domain
+
+import "errors"
+
+// Sentinel errors. Used throughout the codebase for typed error handling.
+// The API middleware maps these to HTTP status codes (SG11).
+var (
+	ErrNotFound          = errors.New("entity not found")
+	ErrInvalidTransition = errors.New("invalid lifecycle transition")
+	ErrApprovalRequired  = errors.New("operator approval required")
+	ErrAutonomyBlocked   = errors.New("autonomy policy blocks this action")
+	ErrConflict          = errors.New("concurrent modification conflict")
+	ErrCircuitOpen       = errors.New("circuit breaker open for target")
+	ErrAbstractType      = errors.New("cannot instantiate abstract entity type")
+	ErrInvalidEdge       = errors.New("relationship endpoint type mismatch")
+	ErrCardinality       = errors.New("relationship cardinality violation")
+	ErrSeedHashMismatch  = errors.New("seed content hash mismatch")
+	ErrAlreadyExists     = errors.New("entity already exists")
+	ErrQuarantined       = errors.New("pattern is quarantined")
+	ErrSkillDeprecated   = errors.New("skill is deprecated")
+)