refactor: split phase3.go + extract MCP tool registry (R4)

internal/httpapi/phase3.go (2627 lines, 12+ resource domains) split into
15 per-resource files:
- actuator.go: SSH execution machinery (initSSH, sshExec, resolveRunTarget,
  executeApprovedAction, jsonErr, gatewayPreflightPassed, resolveTemplate)
- checks.go, classifications.go, executions.go, approvals.go, patterns.go,
  skills.go, approval_rules.go, autonomy.go, risk_classes.go,
  relationships.go, entity_types.go, metrics.go, agent_activity.go,
  helpers.go — one file per resource domain, each with its own imports.

internal/mcp/server.go: newServer (708 lines, 33 inline tool registrations)
refactored to a registry pattern:
- internal/mcp/tools.go (new): toolReg struct + allTools() returning all 33
  tool definitions. Handler logic moved verbatim — no changes to tool names,
  descriptions, schemas, or behavior.
- server.go: newServer is now 9 lines (iterate registry, AddTool each).
  -699 lines.

No function logic, names, or signatures changed. go vet, build, and all
tests pass (httpapi, mcp, db, policy).

internal/httpapi/relationships.go

@@ -0,0 +1,120 @@
+package httpapi
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/dtoro/oikos/internal/db/sqlcgen"
+	"github.com/dtoro/oikos/internal/domain"
+	"github.com/dtoro/oikos/internal/httpapi/gen"
+	"github.com/dtoro/oikos/internal/observability"
+)
+
+// ─── Relationships ─────────────────────────────────────────────────────
+
+func (s *Server) CreateRelationship(ctx context.Context, req gen.CreateRelationshipRequestObject) (gen.CreateRelationshipResponseObject, error) {
+	if req.Body == nil {
+		return nil, fmt.Errorf("%w: request body is required", domain.ErrInvalidInput)
+	}
+
+	sourceID, err := s.resolveEntityID(ctx, req.Body.Source)
+	if err != nil {
+		return nil, err
+	}
+	targetID, err := s.resolveEntityID(ctx, req.Body.Target)
+	if err != nil {
+		return nil, err
+	}
+
+	attrsJSON := []byte("{}")
+	if req.Body.Attributes != nil {
+		attrsJSON, _ = json.Marshal(req.Body.Attributes)
+	}
+
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer tx.Rollback(ctx)
+
+	_, err = tx.Exec(ctx, `
+		INSERT INTO relationships (source_id, target_id, type, attributes, valid_from)
+		VALUES ($1, $2, $3, $4, now())`,
+		sourceID, targetID, req.Body.Type, attrsJSON)
+	if err != nil {
+		if strings.Contains(err.Error(), "unique") || strings.Contains(err.Error(), "duplicate") {
+			return nil, fmt.Errorf("%w: relationship %s:%s:%s already exists",
+				domain.ErrAlreadyExists, req.Body.Source, req.Body.Type, req.Body.Target)
+		}
+		return nil, err
+	}
+
+	rel := gen.Relationship{
+		Source:    req.Body.Source,
+		Target:    req.Body.Target,
+		Type:      req.Body.Type,
+		ValidFrom: time.Now(),
+	}
+	if req.Body.Attributes != nil {
+		rel.Attributes = req.Body.Attributes
+	}
+
+	actorType, actor := actorInfo(ctx)
+	if auditErr := observability.Audit(ctx, sqlcgen.New(tx), actorType, actor, "create",
+		nil, "POST", "/api/v1/relationships", "",
+		map[string]any{"source": req.Body.Source, "target": req.Body.Target, "type": req.Body.Type}); auditErr != nil {
+		return nil, auditErr
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return nil, err
+	}
+
+	return gen.CreateRelationship201JSONResponse(rel), nil
+}
+
+func (s *Server) EndRelationship(ctx context.Context, req gen.EndRelationshipRequestObject) (gen.EndRelationshipResponseObject, error) {
+	sourceID, err := s.resolveEntityID(ctx, req.Params.Source)
+	if err != nil {
+		return nil, err
+	}
+	targetID, err := s.resolveEntityID(ctx, req.Params.Target)
+	if err != nil {
+		return nil, err
+	}
+
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer tx.Rollback(ctx)
+
+	result, err := sqlcgen.New(tx).EndCurrentRelationship(ctx, sqlcgen.EndCurrentRelationshipParams{
+		SourceID: sourceID,
+		TargetID: targetID,
+		Type:     req.Params.RelType,
+	})
+	if err != nil {
+		return nil, err
+	}
+	if result == 0 {
+		return nil, fmt.Errorf("%w: active relationship %s:%s:%s",
+			domain.ErrNotFound, req.Params.Source, req.Params.RelType, req.Params.Target)
+	}
+
+	actorType, actor := actorInfo(ctx)
+	if auditErr := observability.Audit(ctx, sqlcgen.New(tx), actorType, actor, "delete",
+		nil, "DELETE", "/api/v1/relationships", "",
+		map[string]any{"source": req.Params.Source, "target": req.Params.Target, "type": req.Params.RelType}); auditErr != nil {
+		return nil, auditErr
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return nil, err
+	}
+
+	return gen.EndRelationship204Response{}, nil
+}