refactor(workspace): extract layered resolver pipeline for workspace path computation

Replace inline workspace path computation in loop_context.go with composable
WorkspaceLayer pipeline (tenant → team → project → user/chat). Each layer is
a pure function that appends a path segment or is a no-op.

- New workspace_resolver.go: ResolveWorkspace, TenantLayer, TeamLayer,
  ProjectLayer (future), UserChatLayer, SanitizePathSegment
- 16 unit tests covering all layer combinations
- Migrate loop_context.go, loop_history.go, team_tasks_mutations.go
- Move sanitizePathSegment from agent to tools package (exported)
- Zero behavior change — identical paths for all scenarios

internal/tools/workspace_resolver.go

@@ -0,0 +1,80 @@
+package tools
+
+import (
+	"path/filepath"
+	"strings"
+
+	"github.com/google/uuid"
+
+	"github.com/nextlevelbuilder/goclaw/internal/config"
+)
+
+// WorkspaceLayer transforms a base path into a scoped path.
+// Returns base unchanged if the layer is not applicable (no-op).
+type WorkspaceLayer func(base string) string
+
+// ResolveWorkspace applies layers sequentially to produce the final workspace path.
+// Each layer either appends a path segment or returns base unchanged (no-op).
+func ResolveWorkspace(base string, layers ...WorkspaceLayer) string {
+	for _, layer := range layers {
+		base = layer(base)
+	}
+	return base
+}
+
+// TenantLayer scopes to tenant subdirectory.
+// Master tenant is a no-op (backward compat — returns base unchanged).
+func TenantLayer(tenantID uuid.UUID, slug string) WorkspaceLayer {
+	return func(base string) string {
+		return config.TenantWorkspace(base, tenantID, slug)
+	}
+}
+
+// TeamLayer scopes to team subdirectory: {base}/teams/{teamID}.
+// Nil teamID is a no-op.
+func TeamLayer(teamID uuid.UUID) WorkspaceLayer {
+	return func(base string) string {
+		if teamID == uuid.Nil {
+			return base
+		}
+		return filepath.Join(base, "teams", teamID.String())
+	}
+}
+
+// ProjectLayer scopes to project subdirectory: {base}/projects/{projectID}.
+// Nil projectID is a no-op. Reserved for future use.
+func ProjectLayer(projectID *uuid.UUID) WorkspaceLayer {
+	return func(base string) string {
+		if projectID == nil || *projectID == uuid.Nil {
+			return base
+		}
+		return filepath.Join(base, "projects", projectID.String())
+	}
+}
+
+// UserChatLayer scopes to per-user or per-chat subdirectory: {base}/{segment}.
+// Empty segment or shared=true is a no-op.
+// The segment should already be sanitized via SanitizePathSegment if it contains user input.
+func UserChatLayer(segment string, shared bool) WorkspaceLayer {
+	return func(base string) string {
+		if shared || segment == "" {
+			return base
+		}
+		return filepath.Join(base, segment)
+	}
+}
+
+// SanitizePathSegment makes a string safe for use as a directory name.
+// Replaces colons, spaces, and other unsafe chars with underscores.
+// Used to convert userIDs and chatIDs into safe filesystem path segments.
+func SanitizePathSegment(s string) string {
+	var b strings.Builder
+	for _, r := range s {
+		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '-' || r == '_' {
+			b.WriteRune(r)
+		} else {
+			b.WriteByte('_')
+		}
+	}
+	return b.String()
+}