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/agent_activity.go

@@ -0,0 +1,90 @@
+package httpapi
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/dtoro/oikos/internal/httpapi/gen"
+)
+
+// ─── Agent Activity (stub) ─────────────────────────────────────────────
+
+func (s *Server) QueryAgentActivity(ctx context.Context, request gen.QueryAgentActivityRequestObject) (gen.QueryAgentActivityResponseObject, error) {
+	limit := clampLimit(request.Params.Limit)
+	from := time.Now().Add(-24 * time.Hour)
+	if request.Params.From != nil {
+		from = *request.Params.From
+	}
+	to := time.Now()
+	if request.Params.To != nil {
+		to = *request.Params.To
+	}
+
+	var agentID *string
+	if request.Params.AgentId != nil {
+		a := *request.Params.AgentId
+		agentID = &a
+	}
+	var activityType *string
+	if request.Params.ActivityType != nil {
+		a := string(*request.Params.ActivityType)
+		activityType = &a
+	}
+	var entityID *string
+	if request.Params.EntityId != nil {
+		a := *request.Params.EntityId
+		entityID = &a
+	}
+	var cursorID *int
+	if request.Params.Cursor != nil && *request.Params.Cursor != "" {
+		if id, err := parseIntOrZero(*request.Params.Cursor); err == nil && id > 0 {
+			cursorID = &id
+		}
+	}
+
+	rows, err := s.pool.Query(ctx, `
+		SELECT id, ts, agent_id::text, session_id, activity_type, tool_name,
+		       entity_id::text, input_summary, output_summary,
+		       duration_ms, token_count, success, correlation_id
+		FROM agent_activity
+		WHERE ts >= $1 AND ts <= $2
+		  AND ($3::text IS NULL OR agent_id::text = $3)
+		  AND ($4::text IS NULL OR activity_type = $4)
+		  AND ($5::text IS NULL OR entity_id::text = $5)
+		  AND ($6::bigint IS NULL OR id < $6::bigint)
+		ORDER BY id DESC
+		LIMIT $7`,
+		from, to, agentID, activityType, entityID, cursorID, limit+1)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	items := []gen.AgentActivity{}
+	for rows.Next() {
+		var a gen.AgentActivity
+		if err := rows.Scan(&a.Id, &a.Ts, &a.AgentId, &a.SessionId,
+			&a.ActivityType, &a.ToolName, &a.EntityId,
+			&a.InputSummary, &a.OutputSummary,
+			&a.DurationMs, &a.TokenCount, &a.Success,
+			&a.CorrelationId); err != nil {
+			return nil, err
+		}
+		items = append(items, a)
+	}
+	if rows.Err() != nil {
+		return nil, rows.Err()
+	}
+
+	var next *string
+	if len(items) > limit {
+		items = items[:limit]
+		lastID := fmt.Sprintf("%d", items[len(items)-1].Id)
+		next = &lastID
+	}
+	if items == nil {
+		items = []gen.AgentActivity{}
+	}
+	return gen.QueryAgentActivity200JSONResponse{Items: items, NextCursor: next}, nil
+}